With the 3.54 rear you'll see 20mph/1000rpm in 4th-- too high revs with today's gas prices. Keeping the 3.31 will get you 23mph/1000rpm--or 3,000rpm at 70mph--a better ratio for the 4-speed. The 3.73 will get you plenty of oomph in the lower gears without too high revs at highway speed (I drove one for a while with that combo--but gas was at $1 a gallon then) but the 164 has enough torque-- it would probably work ok in od even with the 3.31 (later 240's get away with the 3.31 and od--gutless, but they do get better mpg's) gears. I've also owned other 164's--a '69 carbed 4-speed, and a FI'ed auto '73--the FI'ed '72 w/M410 was my favorite but I wouldn't want to go back to the days of 20 mpg--even my VolvOldsmobile carbed V8 with 3-speed auto did better than that after I changed out the original 3.54 (265 auto body) for a 3.31 from a 164. $1000 seems like a steep price to pay to get the od, but if you do any long distance driving it would probably be the most satisfying to own.