You do know that when you get 300+ HP going thru a transaxle that you have to get a pretty beefy piece of kit. Porsche or Audi ($$$) Less HP, or torque is less money-- Air cooled VW Beetle (turned around) or Subaru. If you don't mind a transverse engine, with probably not much power, you can convert some sort of Toyota FWD, and stick it in the back.
https://www.youtube.com/watch?v=LoA_ZwUWat8
With a tubular spaceframe, you should have all the suspension pickup points thought out already so you DON'T end up with suspension rod ends pointing at (and attached to) the middle of a tube.
Or you could wait a little and get more money and buy a used low end mid-engined car that's ready to go.