Jeffe, I'd be concerned about using the '54 and earlier GM enclosed driveline rearends in any V8 application. You would have to adapt the newer engine to an older trans that fit the enclosed driveline, or adapt a newer trans to the old enclosed driveline sytem.
Best to find a newer rearend that allows an open driveline, and not use your present setup. I would measure your spring perch centers, and the width of the rearend from drum surface to surface. You may most likely need new spring perches, so overall width is really the most important measurement. Then start looking around the wrecking yards to see what might interchange, and various bolt patterns. I'd guess your's is still 6 bolt, and you may have to either convert the front to match a 5 bolt patterrn, or use different bolt pattern for the rear.
An S10 rearend with 5 on 4.75" pattern often works well in these older Pontiacs width wise, and the 7.5" 10 bolt has been used behind V8's with good success.
Here's a rearend width chart for donor rearends:
Rear End Widths
Since you're going with a 700R4, you'll have overdrive, so try to find a S10 rearend (or whatever rearend) with a ratio around 3.50-3.73, as that will work best with the 700R4's low 1st gear, and overdrive.