My 62 w/235 is mostly stock except headers, hei, O.D. and radials. 3:90 rear. 17 mpg @ 70-75 mph all day. 60-65 on the steepest hills.(Sea level, haven't tried it at high altitude)RPMs are 2300-2500 at that speed in 4th over. The O.D. and radials are key. A mild cam and a dual intake(or a clifford intake w/ a 4 barrel) will give you a little more as will increased displacement. I've been told that the porting and polishing the head gives very little gain and then only at high rpm. In other words, a waste of $ since it is expensive. The larger powerglide intake valves (1.94)have been recommended to me with a 4 angle valve job and backlash the seats.
I'm doing all the above to a stroked out 261 because I want to tow and I like the looks a hopped up inline. I'll probably stick w/ a stock 261 cam. But for just driving on the highway, the 235 does just fine.
The t-5 is a good choice for cruising and highway driving, but doesn't work out as well for towing as your 4speed would w/ an expensive gear vendors O.D. I have 7 usable forward gears. Did I say I'm biased?
