early a 5 speed converson

What is your existing rear end gear ratio now?

You could run a 64 - 65 4 speed with the 3.09 first gear and a tall rear end gear and maintain the ball and trunion setup. Any decent driveline shop can make you a slip shaft ball and trunion drive shaft.

Heck, you may even be able to just use your existing drive shaft setup on the swap. It also appears from this article you may be able to use your existing bell housing!?

Then you can have the net effect of an overdrive via the gear ratios in the transmission and rear end.

The early 4 speeds are around and cheap too. The only mod is to have a bearing retainer adapter ring machined out or possibly buy one online (if you plan to use a 75 up slant 6 bell housing).

1964 thru midyear 1966 A-body 3.09:1 - 1.92:1 - 1.40:1 - 1:1 = G/RG and LA engines. (These all used a 4.35" IBR.) 1964 thru 1970 B/RB engine 2.66:1 - 1.91:1 - 1.39:1 - 1:1 = (The "close ratio" V8 gearbox)
1970 T/A & AAR 340s 2.47:1 - 1.77:1 - 1.34:1 - 1:1, = 1970 thru 1974 high performance
(A 2.44 first gear was also produced in this time period and some units had 1.91:1 second gears.)
1975 thru 1987 Overdrive 3.09:1 - 1.67:1 - 1:1 - 0.73:1 (Trucks used a 0.71:1 fourth)

here is the full article:

http://www.slantsix.org/articles/4-speeds/ODA833fourspeed1.htm