The porsche/vw gearbox is going to be your best bet for a transaxle. You could also use the transaxle out of the C6 vettes and make an adapter plate.
Why not use something lighter for a tube frame car? The guys on the Lotus seven forums are making 350rwhp and doing 0-60 in 2.8 seconds using either the sr20. Their cars weigh in at around 1300lbs only and i can tell you they are a hell of a lot of fun to drive and go around corners very well considering they have no down force to speak of.
I hate to post a link to an outside forum, but i believe this might be helpful to you, the guys on this forum know a TON about suspension design, considering most of them are engineers. The link below is a mid engine ls1 powered tube frame chassis.
http://www.locostusa.com/forum...=2857