The longest bar available for the .325 chain that comes with the MS290 is 20". Trying to mate a long bar to it is like hooking a 35' fifth wheel travel trailer to a six cylinder 1/2 ton. It can be done, just don't ask the combination to climb a mountain pass.
Better to hire a professional to deal with a tree that large.

+1 on canguys comments. That would be a very expensive bar and chain combo that would not work very well on that saw.

Don't get yourself into trouble over a tree. Find a friend with experience or hire it out.

I already have a larger chain (I think it's a 3/8") for the larger bar. I am only going to use the larger bar and chain to cut down this one, a trunk about 6 feet high. I will then continue using the 18" bar with .325 chain. I can get the bar for approximately $100.00 It's a mahogany that I plan to use for woodworking.

You will also need a new sprocket for the different pitch chain. Otherwise it would be like trying to put a motorcycle chain on a bicycle.

You should be able to get that 6' section down, but still be careful.

What kind of woodworking? Turning, carving, board? If it will be milled, how are you going to do that?

Boards. Going to take it to a nearby mill. Do you know how much a sprocket would go for?

The sprocket price will depend on what brand you buy and also the quality level.

There are two types of sprockets, rim and spur.

Spur is a one piece system, typically cheaper up front, but higher cost in the long run.

Rim is a two piece system, typically more expensive up front but cheaper in the long run.

Check out the Oregon sprocket to see what I am talking about.

I think costs will be ~$20 on the low end and I am guessing $40 or more on the high end.

If you do not plan to run alot of 3/8 chain or do alot of cutting over the years, I'd stick with a spur sprocket. Simple, inexpensive and works just as well as a rim sprocket.

The rim sprocket is a better choice if you go through a few sprockets per year and you need to be able to carry spares with you when you are in the field.

Good luck on the milling. Get some pics if you can. There is a good milling forum on ArboristSite. Lots of good info and you can post pix there if you can get some. They really like seeing any aspect of the wood process from standing to finished pieces and machines.

The cost of a spur sprocket versus a rim and drum is not that great initially but it is much cheaper over the long run to run a rim and drum. mmra80 should check to see what he already has because his pitch change may be as simple as a $6 Stihl sprocket and the price of the bar, or in Puerto Rico his saw may have come from the dealer outfitted with a 3/8 bar and chain.

There are pros and cons to each type of sprocket, for example, spur sprockets seem to be be case hardened a little more than rims but you have to also understand that once a spur is worn out you also throw away the clutch drum. As previously mentioned, pitch changes with a rim are quick, simple, and cheap and rim sprockets do support the chain better than a spur which results in less chain stretch on saws more powerful than a Wallyworld Poulan.

I wouldn`t recommend that anyone run a long bar on an MS290 for too long unless they are trying to kill it. Make sure that you keep your chain sharp and engine and filters very clean. Also run your fuel/oil ratio at 40:1 for heavy cutting and stop and let the engine cool for 30 seconds or so after a minute of heavy cutting. You do this by unloading the saw in the cut and revving the engine, say one third throttle, so that the fan pushes air over the engine. There is good reason why Stihl sets these saws up to run lighter duty cutting gear for a saw that is purportedly producing nearly 4hp.

No.

It lacks the power necessary to pull such a bar/chain combo and it also lacks the oiling capacity necessary to keep a bar and chain of that length adequately oiled. And if that wasn't reason enough to dispense with the idea, I suspect that you'd end up cracking the plastic chassis when you tried to hoist a bar of that size!

A 290 is about at the end of its usefulness with a 20" bar. It is the right tool for some jobs, but definitely not the job you have in mind for it.