T86AA/D20 to SM420/D300

...In a 69 CJ. I don't know much about the Dana 300, but I see (Craigslist) it's a centered output like my 20. It's mated to the 420 with an Advance Adapter. Would it work, lengthwise?

 

Dana 300 - http://www.novak-adapt.com/knowledge/model_300.htm

If you already have a D20, the 300 would be an upgrade. You realize that a '69 CJ had a Dana 18 from the factory? To have a Dana 20, you need both the 20 and a centered rear axle. Length of the two transfer cases is similar. I would guess that the OAL with the new transmission will be no longer than the current combo. The trans-bell adapter is eliminated, and the SM420 bolts directly to the GM bell.

 

Going to have a short rear driveshaft. Looking at numbers, the T86 length is about 8.8" and the D20 about 10.5". The lengths for the replacements is 10.7" for the SM420, probably 5.25" for the adapter, and either 10.5" early, or 11.625" late D300. So you are going from something a bit over 19" to something close to 26.5". My guess is that you will have 13 to 14" for the rear driveshaft. Will it work? Maybe, if there isn't a big lift. You could move the engine forward but in addition to the engine mounts, the grill and radiator supports will also require not insignificant modifications and the shifters will all move by the same amount. A lot of work.

Personally, this is why I think the SM465 makes a much better choice for this as you can get a very short adapter for it.

 

Forgot to include that. So that would add about 2.5" to the rear driveshaft getting it into the 16 to 17" range and that should work ok. You still have a high Tcase output so a double cardan joint would be highly desirable on the rear. Personally, I don't think the D300 is that much stronger than the D20. My biggest problem with the D20 is the crappy stock low range ratio. Easily cured but that take $.

 

I think the big advantage of the 300 over the 20 is the lower low range.

 

Ok thanks....Probably gonna go pick it up tomorrow

 

The 2 main advantages of a 300 over a 20 is in stock form the low range (2.03-1 for the 20, 2.62-1 for the 300) and the better aftermarket support for the 300.

 

The 2 main advantages of a 300 over a 20 is in stock form the low range (2.03-1 for the 20, 2.62-1 for the 300) and the better aftermarket support for the 300.
There are also shift rail mods you can do to the 300 to give more options you can't do to the 20 but unneeded for most people.