Trans swap

Plan on swapping out my M21 for a SM420 as soon as I can scrounge up the $$$ for the Novak adapter. Anybody know if I'll need to lengthen/shorten the driveshafts? I don't have the SM420 at my place to measure things out.

Any other info on the swap would be great. Thanks

 

Check this out:

http://www.earlycj5.com/tech/trans/SM420/sm420_swap.shtml

 

I had an M22 which is about the same length as the M21 and they were quite a bit longer than a 420. IIRC almost double.
As for the driveshafts, figure *around* 16" for the rear, and 28" for the front. Your mileage will vary due to lift, engine position, center support position etc, transmission height, etc.
I have even read where the rear came out closer to 15".

 

Thanks guys.

Just found a site claiming the M21 is 21.5". Big difference from the SM420. Looks like new drivelines are in my future. I wonder if the bellhousings are similar in length?

 

I think the 420 and muncie M21 share the same bolt pattern, however I think I recall someone saying you have to access the bottom bolts from inside the bellhousing or something. I know with my 465 it just bolted up from the outside. Standard GM pattern.
I think the 21.5" for the M21 is with the tailshaft housing still attached. IIRC the saginaw 4 speed (which is the same length as the M21) I removed from mine was about 11" long. 12 inches long with the AA 1" adapter installed.
I did have to modify my driveshaft, but I also did the spring over which affected it too. Look on the positive side though, you can shorten the existing shaft,(cheaper) but if you need it longer you'll have to make a new one (expensiver)