Thanks for clearing up the question about the transmission type, although ratio-wise they are similar and in 4th, identical. I can tell you that a change from 4.3 to 3.9 will be enough to feel a difference at cruising speed. I built my first version of my VolvOldsmobile (aluminum 215 V8) on a 1976 265 body (originally a 3.54 ratio) and later swapped in a 3.31 from a 1975 164. It made a noticeable difference at speed. Now the V8 sits in a '93 245 (an auto equipped car w/3.73) but now I have a Camaro 5-speed installed so top gear is an OD ratio. If your car truly has an M45 it would not be a stretch to find an M46 as a bolt in swap (swap to the B20A bellhousing) that would provide you with the tighter lower gears but allow lower highway rpm due to the .80 OD ratio. Going from a 4.3 to a 3.9 would give you a 10 percent drop across all the gears. Going to a 3.73 would be more effective in top with the M45 but would be a lot for the lower gears. I'd go the M46 route if possible. -- Dave