A 300ZX has the same engine as the J30. Although there is a twin turbo version as well as a manual transmission.
I must say that's the first time Ive heard someone say a J is big and bulky.
Although my oldest son has a Jetta so I think I get what you mean.
I think a 240/280 Z is what your after.
As Swimshark said the J30 Engine would be tough to fit.
Ive heard the RB Series engine is pretty healthy as is.
A smaller body car that's MUCH lighter.
The J is a lot like the Dallas Cowboys.
You either Love them or Hate them.
