I have a 1956 Bel Air with a 350 and the original 3 speed manual trans that has been moved from the column to the floor.  I would like to put in either an automatic or a 4 speed but it seems like the local guys are resistant to the idea.  They like the idea of just rebuilding the trans I have.  What is best for me to do?  I don't have tons of disposable cash so the idea has to be reasonable.  THanks so much.  

being as it already has the clutch set up,i would go for the 4 speed. the bad news is,that its hard to find any bargains on a good muncie or borg warner 4 speed these days,and you would also need a new shifter.you might try ebay or you local car clubs for help finding one,but expect to pay $600+ for a good one(if you find one for less,buy it!)an automatic might be cheaper in the long run,but not as much fun!and you can probably sell your original trans to somebody restoring a 55,56 or 57 chevy,and recoupe a fair portion of your money.  

With the popularity of 5-speeds today, the supply of four speeds has gotten better, and so have the prices. I've been seeing more of them at swap meets.

I agree, since you've already got the clutch pedal setup, go with a 4-speed. Do some hunting and I'm sure you can find an M-21 or T-10/Super T-10 for your car. You might even get lucky and score an M-22, you never know.