I did the same thing when a B20 went into my 544. Firsst I changed out the whole rear axle to a newer 544 that meant from a 4.56 (1960) to a 4.10 (guess) from a 4 speed car. Then I still was revving too high for highway work so I placed a M41 with overdrive. A couple of years later, (winter driver) I chose to replace the whole floor/frame from the front seats back with one from a 1800ES. This included disc brakes and heavier springs and a flat floor inside. but I now had a 4.30 ratio. Since I really drive this car all winter long and to the tune of over 15,000 per year, I had a rear axle from an automatic with 3.91. I went with that and I beleive I have the lowest gearing possible....M41 with O/D and the rear axle from the automatic 1800.

http://www.georger.com/virgil.htm

Christopher