Is there a junkyard gem slant six combo?

All factory slant six pistons will be the same CR. There are some different aftermarket piston configurations within the "stock" description. Some have a slightly lower compression distance. There are also aftermarket "export" low compression pistons. Be careful you don't get these. For what you are wanting to do, Any 225 block, pistons, and rods would be essentially the same. The main thing you need to be aware of is the change from the small to large crank rear flange between 1967-1968. Also the change from forged to cast crank in 1976 (the crank and block changed). 1980 and 1981 engines you have were hydraulic lifter engines. Solid lifter engines and hydraulic lifter engines can be converted in either direction by swapping the complete valve train (cam, lifters, pushrods, rocker arms, rocker shafts). Be aware that the factory used a steel shim head gasket, and the aftermarket only has a thicker gasket available, so you would lose compression if not milling the head or cutting the deck to compensate. If cutting the deck or milling the head, I recommend cutting more than the minimum required to increase the CR higher than stock. Up to a certain point there is no downside to raising the CR. It will help both power and fuel milage.