3600 = open driveline - if you are married to the idea of a modern six, forget using the stock bellhousing and cross member.
You likely have 4.56 gears or so - swap in a 4.11 from a 67..72 3/4 ton chevy pickup. you only need to change the center pumpkin - it is a bolt in.
Then get a 250 L6 with a manual transmission and make a new rear cross member that goes below the tranny tail shaft. You will be much happier with the driveability of the truck with this setup than with the existing 216 + high gear ratio rearend.
If you really want to keep the bellhousing / transmission that you have - get a 55+ year 235 or 261 and do the differential gear swap as previously indicated - alot easier and mostly the same benefits.
