[Wayne Ryan]

I have a ram cobra with a 350 Chevy motor approx 350hp, a TH400 transmission and a jaguar rear. On the autobahn (Germany) I travel at 140 kph at 4000 rpms. I would like to know if there is a transmission which has the same outer dimension but lets me run at this speed at lower revs that I can replace without any major changes to my car?
Thanks for any help here!!!!!!!!!!!!!!!!!!