You have two choices. You can either remove the carburetor/tank combination or get a different type of muffler. The carb is held to the block by two 1/4-20 screws. The usually have 3/8" heads and are also slotted. Also, the oval muffler is the quieter of the two.

The other muffler is round and smaller and is often called a "hotdog" muffler. With this muffler you can cut off the old muffler with a hacksaw and unscrew the nub. Although saves having to remove the carb, the "hotdog" is louder. Also, with the "hotdog", you don't need to worry about a locknut. Since the muffler is round, you simply screw in until tight. Below is a "Hot dog" muffler:

Here is the larger "oval" muffler:

One last question, I have searched the net for days so I wouldn't have to ask but cant find the answer. Can the Tank and Carb be removed as 1 piece? I see the 2 carb mounting bolts but looks like I only have access to the one on top. Guess if I have to split the two I better start looking for gaskets. Thanks again

Normal procedure is to remove the carburetor and tank as an assembly, and then split the two apart on the bench. You may need an angled screwdriver to break the lower screw loose. From there you can use a smaller screwdriver at an angle to wobble the screw out. Lastly most Vacu-Jet carbs (small tank) are only mounted by the carb screws, but some have a strap from the bottom of the tank to the lower engine block. If so, you will need a 3/8 hex socket. Picture is an angled screwdriver for reference.