You are probably better off to use one of these type quad breakers IF your panel is listed for them.

http://www.acdcusa.com/mp22020-murray-20-220-20-quad-murray-mp-circuit-breaker/?gclid=Cj0KEQiAjMC2BRC34oGKqY27jtkBEiQAwSXzfrEyeQxfwXANgkFyxMFQLGiwwwFsUyvWAf73LAPAKTsaAhx68P8HAQ

They only take up two spaces for two 20 amp circuits and a 240 amp circuit from the center two handles.

NO. You do not need to redistribute the breakers. Every other breaker on each side of the panel is already an opposite side of the 240. That is how the panel splits up the 240 into 120, and then can go back to 240 by putting in a double pole breaker anywhere in the panel. I usually put the higher amp breakers (30&50) closest to the main lugs or breaker, and arrange the rest of the breakers so they are grouped to where they go in the house...basement breakers on the bottom, kitchen at the top by the range breaker....just easier to find.

I hope you don't have a 2,000 amp heater. That's going to be one heck of an electric bill. ;-)

Just replace the 50 amp breaker with the 20. If there are still wires going to it you can either pull them out of the panel or cap them off and neatly tuck them out of the way.

BTW, you may need arc fault protection for the general use circuits as well.

I know it is wrong to not use AFCI breakers now days, but just putting afci on the 3 new circuits without changing out the whole panel isn't any better than playing Russian Roulette, IMO. Especially, since you would be putting the AFCI on the new wires instead of the potentially old ones that may actually need it. If you have smaller than a 40 circuit panel, no AFCI breakers at present, and may have plans in the future for more electrical such as a hot tub or landscape lighting, then now would be a good time to at least think about the bigger picture, and either change to a larger panel or add a subpanel. BECAUSE, by the time you change all the breakers to AFCI, the time and money spent won't warrant not spending the extra $200 on the new panel.

Otherwise,

210.12(B) Branch Circuit Extensions or Modifications -- Dwelling Units.

In any of the areas specified in 210.12(A), where branch-circuit wiring is modified, replaced, or extended, the branch circuit shall be protected by one of the following:

(1) A listed combination-type AFCI located at the origin of the branch circuit

(2) A listed outlet branch-circuit type AFCI located at the first receptacle outlet of the existing branch circuit

Exception: AFCI protection shall not be required where the extension of the existing conductors is not more than 1.8 m (6 ft) and does not include any additional outlets or devices.

...

210.12(B)(2) Would be more in line with the actual benefit of an AFCI in this situation, since protecting one cable all the way back to the panel while the other ten old cables that it is run next to are not protected is a bit futile. I would also add a blank face AFCI at the beginning of the lighting, or other circuits that don't start with a receptacle. You could also put the old circuits on the quad breaker, and then put the new circuits on a full-spaced AFCI breaker if you want to please the inspector with a code run-around ;D

A lawyer must write those codes. No plan on putting any AFC's in. There is one in my electric panel though. A contractor told me that he puts them in to pass code and then removes them because they get so many call backs with them tripping---too sensitive. Anyways, it seems that with the double 20 amp breaker dedicated to the heater that there would be room to add something else to it in the future---lights in another presently unfinished portion, for example. So Fred, are AFCI's required on any new wiring? If so, why do stores still sell the other breakers? Or, are they just required in certain areas as the GFI"s are?

One thing to remember about distributing the loads on either side of the panel is that you don't have all that stuff running at max amps at once anyway. So about the best you can do is have approximately the same total amps of 120V breakers on each side of the panel. It's rarely going to be even in actual use anyway.

"Sides" don't have anything to do with distributing the loads. You could put breakers all the way down one side of the panel, and leave the other side empty, and still even out the loads between phases.

Sides lol

"I know it is wrong to not use AFCI breakers now days, but just putting
afci on the 3 new circuits without changing out the whole panel isn't
any better than playing Russian Roulette, IMO."

This makes absolutely NO sense. "Russian roulette??? In what way?
What does a new or old panel have to do with installing AFCI's? As long as the panel will accept them.

If you are not going to bother reading the rest of the post that goes on to explain the comment, I am not going to repeat it here.

"If you are not going to bother reading the rest of the post that goes on
to explain the comment, I am not going to repeat it here."

Thanks, but I DID read the rest of that post and was even more confused. I just didn't get the silly "Russian roulette" comparison.
If you are saying why bother putting in AFCIs now because if you change the panel later they might not fit, then you have an odd way of explaining things.

Let me come at that from a different angle. I am only talking about protecting the wire between the panel and the first junction box of the circuit, where an AFCI receptacle can be installed easier if the panel is already full of standard width breakers, but will accept a quad breaker as I first suggested.

If I have five OLD panel runs through the floor joists that are not AFCI protected, and add a sixth panel run along side of them that IS AFCI protected, then it is like only taking one bullet out of a revolver to play Russian Roulette with.

Unless you are going to replace all the breakers, instead of just the circuits you are working on, the old wiring in the joists is statistically more likely to have a problem that the AFCI would detect.

Thus, the reason for 210.12(B)(2), even though, it doesn't technically apply to a new branch circuit in an old panel.