spawpoet

While I haven't swapped one before, I don't believe you have to crack open the diff to remove the flange. If I'm correct you simply remove the bolt on the front of the diff snout, and use a puller to remove the snout. The new flange should just press right in.

I would guess you have a vacuum leak on the rear carb. could be through the throttle shaft if it has worn excessively, but also check that all vac plugs are in place, and that all gasket surfaces are fully sealing. Also have you checked other potential issues such as plugs and valve lash?? I've had a cracked/non-functioning plug on a couple of occasions.

Decking the block will raise, not drop compression. You are de facto reducing the volume of the combustion chamber.

+1. Especially if this motor hasn't run in awhile.

If you can do a leak-down test, you may be able to determine whether you are losing compression at your rings, or your valves. Also like the poster above mentioned you can throw some oil in the bad cylinders. If the compression rises significantly you need rings.

I'd do a full, proper tune up on the car including tuning the SU's. Set the timing, valve lash, make sure the plugs and wires are good and in the right order, and make sure you are getting proper delivery of fuel to the carbs. Once all that is done then properly tune the SU's. Most likely the car just needs a good tune-up. Is the compression good on the motor??

How do your sparkplugs look?? Are the SU's in a good state of tune and at the proper mixture?