Is it possible to beef up a TH350 to handle up to around 500 hp? I already have the 350 and the driveshaft. I don't really want to buy another tranny and it will be largely street useage. Not planning on a strip at all, but wanna punch it at the lights and make it move.

Any thoughts?

Yes it is. I would suggest having the 4L60E center support & sprag installed, and cutting the direct piston cut down to increase the clutch pack to 5 frictions. And also you need the HD intermediate sprag race. It really all depends on how much $ you have to spend on parts. There are several parts available from different ratio planetaries, hardened shafts, and lug sprag drums, manual valvebody, trans brake, etc.