Slave cylinder (all Volvo #'s) 653117 is for the b16 engined Amazon, 669782 is for the b18 up to 66 and is 3/4", 673030 is for the rest of the hydraulic clutch Amazons and is 13/16". The pushrods are specific to the slave cylinder.
On the other hand, the clutch, cover and t/o bearing have no relation to the engine or clutch operating system. They just need to be type-matched (F&S style or B&B style).

I really think you have a syncro problem from the fact you have 3/4 issues and not 1/2 issues or issues with all the gears which would be more indicative of a clutch problem.

Tearing down and rebuilding the tranny is not a big deal (o/d tosses some complication into it) and few special tools are needed. The only critical tools you are unlikely to already have are the monster eaton ring pliers (snap ring holding 3rd gear on the shaft... will have to come off) and a layshaft pilot/dummy. The snap ring pliers are truely important to buy quality but the layshaft dummy can be made from 1/2" copper pipe (rigid style or "red" copper) and a hack-saw.

You will need to order dog ring springs and replace those no matter what other troubles arise (trust me). The actual dog rings and selectors are pretty rugged so the brass syncro cones are the most likely failure source (OK, bearings are too... does it "rumble" in gear?). Oh, get the seal/gasket kit and beware the rinky-dink itty-bitty o-rings supplied to seal the input shaft cover screws... soak-em in motor oil before using at least overnight as they like to crack. Only install them when it is truely time to assemble!

But before you do that, run the selectors back and forth with your fingers with the cover held on by hand first... This is the time to tell if you mis-assembled anything!

Mike!