May I suggest that you find a 5/6 speed manual transmission to mate up to to your 250 build up. Even though a 6 will consume less gas cruising around 3500 rpm the high rpms will eventually take their toll on the engine and will allow you to cruise without the turbo(s) running. I did this conversion on my 67 Mustang, bought a T5 with a 0.73 overdrive gear and 3.55 rearend and I'll cruise @2500rpm @ 70mph. You may also want to look into a V8 if not for the sole purpose that you can purchase a crate motor for about the same cost, and in most cases it will be cheaper than a 6 buildup incase cost is an issue. If your interested go to this web site
http://www.badasscars.com/index.html