I'm having the same dilemma right now. I have a SPF coupe on order and just ordered the motor - a Keith Craft 427w. I wanted to put in a TKO600, because I heard that in the relative quiet of the coupe it could be a bit noisy. The problem is that no matter which overdrive I pick, freeway cruising speed is either too low or too high with the stock 3:46 Dana rear end (2000 RPM at 74 MPH with the .64 or 2500 RPM at 72 MPH with the .82). I'm focused on that since I plan to use the car on long weekend trips from time to time.
Either way, I'm ordering it from Rockland Standard Gear (
www.rsgear.com). I got a great tip from a VERY reputable source (Badger) on these guys and especially the work they do blueprinting the transmissions. Their prices are great, the blueprinting is relatively inexpensive and makes the unit quiet, smooth and bulletproof.
Doug