John, this engine is in remarkably good condition for the vintage. Considering the use of paraffin motor oil and popular additives like STP during that period, this V-6 shows very little sludge or signs of "death" by the alchemy of heat and high volatility hydrocarbon molecules in the motor oil.
You can have these pistons and pins measured and assessed by the automotive machine shop. If there is no skirt wear or ring groove/land wear, and if the pins still fit correctly, you could even consider using these pistons again if the cylinders do not require a re-bore. Take a well lit close-up photo(s) of the piston skirts, I'll provide my opinion.
The head work will include a magnaflux check for cracks, magnaflux the block, too. I would install hard steel exhaust valve seats to prevent valve seat recession with unleaded fuel. Stainless or hard steel exhaust valves are recommended, too. (We can check on the stock/OEM exhaust valve material. Truck engines of this vintage had hard steel exhaust valves and hard seat inserts in stock form. Not sure now Jeep/GM addressed this engine, it was Buick Special passenger car derived.) The shop will want to deck the heads, remove just enough material to square up. This is all insurance.
Even if the cylinders are still true without a ridge at the top (can't tell from the dark photos), have the block power honed by the machine shop without increasing piston-to-wall clearance beyond tolerance. If the block requires boring, this will dictate new oversized pistons and rings. We can talk about my piston and ring set recommendation. Hot tanking the block and new cam bearings are in order here, regardless of the bore size.
Connecting rods should be "reconditioned" if you change the pistons. The crankshaft will at minimum require light polishing of the journals if still round and undamaged. If worn, you can likely get by with the optimal 0.010"/0.010" undersize regrind with polishing for the rod and main journals. You will want to do the Melling high-volume oil pump that I illustrate in the Jeep CJ Rebuilder's Manual: 1941-71. This extends the pump cavity depth and spaces for the longer pump gears. (You must center the spacer plate carefully as I demonstrate in the book.) Make sure the pump cavity is undamaged.
A new camshaft, new hydraulic lifters, and a new timing chain and sprocket set are standard fare. Like your idea of a master rebuild kit, it should include all of this. I prefer Felpro head gaskets (included in a complete Felpro overhaul gasket set) to eliminate the need for a re-torque on head bolts after the engine is in service.
Given that this engine is odd-firing and inherently "rough as a cob" (Jeep engineer remark from the period), be sure to balance the crankshaft, damper, flywheel (resurface, too) during this work. It pays to balance all reciprocating mass parts and weight-match these parts as well. A quality machine shop should have balancing equipment, and this cost is well worth the expense, especially for this engine design. Install a new crankshaft pilot bearing. Likely you'll be installing a new clutch disk, clutch cover and throw-out bearing.
You have the Delco distributor, a real plus, the Prestolite was terrible. The carburetor is a 2GC Rochester. Tuning and clean-up here is straightforward. The fuel pump needs replacing, especially with the apparent drag and over-tension that caused the cam lobe wear. Use of modern oil and proper break-in should eliminate risk of premature wear like this. I'd be happy to make oil recommendations for break-in and afterward. We can discuss that later.
Given the overall good condition of this engine, I would try to reuse all core parts and not "exchange" the crankshaft or other parts for rebuilt or reconditioned parts that have an unknown history. If your current parts are still standard size, you have pristine core pieces that should be kept and reconditioned.
This is a start...I'm opening with an overview, there are more details to share. I am pleased to discuss the Buick V-6 engine, in odd-firing 225 nail head form or the later 231 and 252 design. I have the Buick Power Source book, four period shop manuals and period parts catalogs for backup reference material...We're covered!
Thanks for sharing your project, John!