There are a variety of gear ratios for the V8 top loader trans, and over the years there has been much mixing and matching of parts and ratios, so check the gear sets on any trans you buy. "39 transmission" has almost become a generic term for any topload V8 trans, even if it has a 37 case, 38 gearset, 39 shifter, 40 synchros.
Any one thinking of putting in a V8 transmission should browse Van Pelt Sales website which has a lot of good info. Here is the link to the gear ratio chart.
http://www.vanpeltsales.com/FH_web/f...gearratios.htm
The MItchell trans with the 15% higher gearset is fairly close to the 28 tooth '39 gearset. Easier to swap in, has synchromesh, looks authentic, you know for sure what you are getting, but more expensive.