Just finished auto swap...
 

Linkage came in from Kugel yesterday and finished up my swap so I can drive it now. Anyways the speedometer gear is off, which I will change later. But for now I was just gonna use the tach to monitor my speed and was just wanting to know if the final drive gear is the same on a TH350 as in the factory 3 speed. And is there any calculators for figuring up the gear I need. I have a 307/350 LWB truck with 4:11 gear and 235/70/15 tires. Thanks.

Is this what you need? http://www.tciauto.com/components/speedometer_gears.htm

try this i used it and was dead on. http://www.cvsr.net/php/links.php
the final drive for the 3 speed and the th350 is 1:1 all the final drives are 1:1 unless it has over drive the the 3rd gear is 1:1
hope this helps

How hard was the swap? What all was involved. I am wanting to do the same on my 72 GMC. Thanks JPJ