RRussellTx - 1974 Commando 850 Project

Vapor blasting is a very effective cleanup and leaves an attractive finished appearance. The Amals were Vpr. blasted then the manifolds, bowls and caps went out for powder coating along with a lot of other parts...much like your project appears.
Once I got a look at the finished results of vapor blasting I ended up taking lot of parts to my local shop. I found the cost very forgiving from the perspective of time spent on cleanup and the need for clean fitting/sealing surfaces.
The guys in the shop would come out to the truck and help me haul boxes inside, we were on a first name basis quickly.

Keep the peace in the house. An outdoor gas grille works well to heat case to remove bearings. With a light tap they should drop right out on their own if case is upside down. Put the new ones in the freezer before hand and install while the case is hot.

Unless I missed something, I would never bore cylinders without the pistons and rings on hand to measure first.

Trying to find set of pistons within a few thousands DIFFERENCE from a different brand, could/should prove to be your worst nightmare...

I like worst nightmares. I bought Molnar barrels with a stock 750 bore and stock diameter MAP Cycle 750 pistons and long rods, measured the ring gap and tossed the pistons in. Engine would have blown up by now if they didn't work. I never bothered to measure the pistons or bores. Well maybe I did, but didn't write it down anywhere. Living on the edge

GP, no problem. I always do what you do, have the machinist estimate what it will take to clean things up. Then I get the pistons and have them measure and bore and hone to tolerance I want. I always used .0045 with the Tiawan JCC type pistons. Even if the bore and hone has been done the pistons and bores can still be requested to be measured. If the tolerance is too tight some additional honing should correct it. If too loose will probably have to live with it. A little bit bigger shouldn't be a problem on a street bike that gets typical vintage bike mileage. I'd be mainly concerned about them being too tight.