To clear up a few things-if there was no vacuum to trans it goes to max line pressure and shifts are at the wot governor set-points , from 1-2 and 2-3 so the 2nd shift would be late also not just the third.
Is reverse solid, if its NOT solid there is a leak in the direct clutch drum (3rd gear drum) or a missassembly problem/accumulator leak. The governor is under the 4 bolt cover on the side of the back of the trans , it can be pulled out and checked in minutes, if you hold the weights in with your fingers and invert it you should be able to see the spool valve in the center drop, when you flip it the other way you should see it drop the other way, you should also see a spring on neach side under each weight Something I have seen cause a similar problem is if someone leaves the detent solenoid inside the trans (bolted on with 2 bolts) loose or leaves gasket out on the old style ( one with aluminum cover on the solenoid windings) it can "bleed off" and make the trans shift later similar to what your describing, Smitty