my first engine rebuild (318) advises

I admire your ambition on running a Mopar in Finland north of the Arctic Circle. It would be like me trying to find used sheet metal for a Saab 96 in Hotter Than Hell, Texas.

It seems to me if you are serious about fuel economy, using a 4-barrel carburetor, bigger valves, and a longer duration camshaft are going to be counter-productive. Higher compression, on the other hand, is good.

If the heads have not been off the engine, the pistons and connecting rods haven't been out either. To remove the pistons, you'll need either a ridge reamer (for a big ridge) or a cylinder hone (for a small one) before you try to extract the pistons. It is very important to get the bore smooth all the way to the top if you are planning to re-use the pistons. If the bore is not smooth, the rings will hang the piston up damaging the place where the rings sit in the piston. Your friend with the tractor repair business probably understands this.

If you are going to re-use the pistons, I'd re-use the wrist pins as long as they are completely smooth. The connecting rods are probably good to go as long as they are not twisted, bent, or cracked. I've re-used wrist pins and connecting rods several times before with no regrets or performance issues. As far as wrist pins for the KB pistons are concerned, you'll have to ask them if they need something special or if the stock pins are good to use.