This would be the time to rebuild it, but I understand that if that's not in the cards, then it's not in the cards. Unfortunately it's not as simple as just swapping pistons. You'd have to buy new pistons, get the block bored, hot tanked, new cam bearings installed, have them check out the crank, grind if necessary, recondition the rods and balance the rotating assy. Adds up in cost quick. Just keep in mind if you skip this part now, someday you will want to go back and redo it.

At least objectively evaluate your heads before investing into them. Weigh what you'll have to invest into them, installing hardened exhaust seats and new valve springs, cleaning heads and valves versus buying aftermarket. At least if you spring for aftermarket heads now, that's something you can reuse later on down the road.

I had decent results with my smogger 440. I found with mine the key was ignition timing. You're going to NEED to recurve your distributor for your new cam so you need to plan for having to do that. I couldn't get mine to run well consistantly until I dialed in the timing correctly.

What rear gear ratio are you going to be running? You need to decide that first before you can pick a cam.