As a new guy here, I've been busy reading and learning, great forum folks. This subject I feel able to contribute something back. I can not make any recomendations without first knowing how the engine was built as far as where your peak torque is. A general rule of thumb, don't cruise lower than 1/2 the rpms your peak torque happens at. Throw in another 100 rpms if using an aluminum flywheel. Most medium high performance crate engines will produce max hp at around 5600 and max torque around 4200 rpms. If using a steel flywheel then 70 mph highway cruise should be around 2100 rpms, 2200 for alumimun. I have had both a high hp, high rpm screamer 355 engine and at the opposite extreme a real grunt master 383. The grunt engine's hp is falling off at 4050 rpms, the torque is 516 lb/ft at just 2800 rpms. I don't expect anyone here has peak torque at just 2800. Because of that, I replaced the Muncie M21 with the TKO 600 0.64 OD 5 speed. My car weighs 400~600 lbs lighter than your Cobras and with 31" rear tires and a Jaq IRS 3.54 I cruise under 1700 rpms at a tick over 70 mph. The fuel mileage has improved from 12 mpg with the 355 to high 20's now. Last 1200 mile trip I saw 24.6, 30.2, 15.9 (got tired easy driving), and 28.4 mpg.
