I just noticed this post. I looked into the same transmission because I wanted to go faster at lower RPM, but wanted to keep everything else stock. It just won't work without modifications. I like the divorced Spicer t-case and didn't want to swap that out and then buy new drive shafts.
Since you are going with a different t-case, it seems like your best bet is to pull an entire power plant, transmission and t-case out of an 80's Blazer and just mock it in.
I think your biggest issue with this is that you still have drum brakes. You really don't want to be going 60-70 and having to stop in an emergency.
In the end, I am very happy I stuck with the SM420, but I can't go on the freeway, which is probably a good thing.
I would love to build another 55 with a complete modern drivetrain with disk brakes, that would be awesome. I think you should do this and ditch the NAPCO front axle and ship it to me!
