Re: Will Dana 20 fit behind 304/TH400?
I did this swap a year or two ago and I've been very pleased with it although it is failry time consuming and gets expensive. You might do a search under my name and see if anything comes up.
The tailshaft of your th400 has to be changed out from the 10 spline shaft that mates to the q-trac to a 30 splined shaft. This requires tearing down the whole tranny, so might as well get a rebuild & shift kit while its out. My ran about $275 with the new 30 spline shaft. (the shaft my tranny guy used was a standard 2WD th400 shaft). The cheapest adapter is the factory adapter from vehicles that ran the th400/Dana 20 combo. 67-73 Jeepsters and old J series trucks had this combo. I paid $300 for the th400/adapter/Dana20 unit out of a wrecking yard. Doesn't matter what shape the th400 is in. The only thing you might use out of it is the tailshaft. D20s are pretty tough so not much to go wrong there, just make sure it shifts into 2hi, 4 hi and 4lo by checking the front output.
It is probably easier to find the dana20 and adapter for sale together as a unit. Most dont want to sell just the adapter, so keep your eyes out for one on the for sale board. You will have to move your skid plate rearward 1-2" for the new set-p. I just redrilled holes in the skid. Both driveshafts had to be changed. Shoter in the rear, longer up front. You'll have to take your measurements once the new t-case is in place. The shift linkage will have to be modified some to have it come through the floorboard in the right place. What you have to do depends on the D20 you use. The Jeepsters and j-series had a remote shifter box, a little different than the standard D20. The tech page has a pic of one.
I guess I'm hard on parts and I've busted one factory adapter already. They are made fron aluminum and have two weak looking footings where the t-case mounts to the skid plate via the rubber mount. The adapter I have now I beefed up by having an experienced TIG wleder weld some additional bracing to the adapter footings. I strongly recommend doing this while you have everything out. It sucks pulling that crap back out.
Dana20s are pretty common at around $50-75 in my neck of the woods. But like I said, I would probably look for the adapter first, you may find yourself buying a Dana20 just to get the adapter.
I like the Dana20 ALOT more than the q-trac. The low range isn't very good (2.03:1). Your q-trac w/low range is lower, 2.46:1? But the cast iron, all gear stoutness of the Dana20 I love. Not much to go wrong.