well, almost everyone on maliburacing.com uses the TH350 or TH400....reason being they either are, or can be very strong trannies....i used to hate my th350 till i found out how strong it was, i'd say keep it, it can certainly handle a TON of power, and it'll give you some good runs, maybe consider a new tourqe converter though.
I've heard good and bad about the 350, but i was wondering about it being a computer controlled lock up converter transmission. I know B&M sells a lock up control module at summit and was thinking about trying that since i no longer have the computer in the car. I've also found a decent lockup converter and a shift kit, but haven't bought anything yet. The tranny still worked fine when i pulled it and i'm not sure if i should stick with it or get a non computer non lockup tranny.
I like the 350, I think it's a good dependable trans that will take a moderate amount of abuse. I guess it would depend a lot on what your plans are for your Elky. Do you plan on dropping in a high horsepower/high torque engine, if so you might want to upgrade to a 400THD. Are you looking for better gas mileage? Then you might want to go with a 700. An automatic trans swap is a fairly easy job. What you can afford now or what you might want to do later can be accomplished when time/money permits. I have a 350THD in my Elky with the original 305 motor, it's worked just fine now and there's 98K miles on the drivetrain. My plans however are to drop a new motor in soon, hopefully a Big Block, when I do, I'll be switching over to a Borg Warner Super T-10 4spd. If it was me, I'd stick with the trans you have until your ready to swap the motor also, then I'd consider something like a 400THD from TCI (streetfighter).