i noticed it said you need a 400 harmonic balancer, but as for the flexplate, can i just throw the one from my previous motor (350) on there or do i have to go find one on a 400 and yank it?

You HAVE to use the 400 one - these motors are externally balanced, unlike the 350 which is internally balanced.

can i buy a cheap aftermarket one or am i gonna have to go hunting?

You do not have to use a 400 harmonic balancer i use a 350 on mine and we had it balanced at the rear of the engine (extra metal added to the end of the crank shaft)