Your best bet is to run it till it dies unless you know somewhere to get spare Piston and Sleeves from. The rod is the same as the rods in newer engines. It does have a different crank and main bearing as well. If the engine is still as tight as you say you should be able to get 10+ gallons out of it if you take care of it! If you want to go the resize route down the road you can just order the newest RB conrod, wrist pin, and clips and replace them when you have it pinched, that will give you some extra life and will allow it to be pinched a little tighter when the time comes.