I overhauled two of those engines a long time ago. Simple, even crude, but tough. Hard to build for high performance, mostly because they have the intake manifold cast into the head, but you can improve them a bit without spending much or any money, if you have some shop-skills and a little time.
The combustion chambers are hilariously rough and malformed, but you can carve away the unwanted ledges and projections with your port-grinding kit. You will remove so much metal that you'll drop the compression, so you'll want to check the volumes and mill the head and block deck accordingly (do the block first, to get a .038-.042" squish, then check the cc's).
The exhaust system is as crude as on most old American-made sixes. I don't suppose there are any aftermarket headers available, but if you or a buddy can weld, that's no problem. J.C. Whitney will sell you the exhaust U-bends. You section them with a hacksaw to get the curves you want while your buddy welds them up. You'll only be out the price of the U-bends, a pizza, beer, and a few lap-dances at the local dive.
The intake can't be made into anything impressive, beyond altering it to take a 2-bbl progressive-opening carb. Oh, you could saw it off, braze some flanges on the head, and weld up a good bolt-on manifold, I suppose. But the engine will make acceptable street torque without going that far.
Johnny, I expect that all you want is to get to the beach and back without car trouble. But if you wanted to have a distinctive ride like no one else's, you sure have the makings of one in that old Rambler. I'm a contrarian, and I hardly ever look at the three dozen SBC cars that show up at the local cruise, or even the smaller number of 340 Mopars, 302 Fords, or other varieties that are so common and so well-supported by the aftermarket. I like the old, the obscure, the wierd! But not restorations; I like creative hot-rodding. My very nice, lightly hopped-up '64 Rambler American coupe (which I gave to my sister), and wagon (which went to my brother), are long-gone now, and I have owned what most guys would say are much cooler cars in the intervening years. But I wish I had hung on to one or both of the old Rumblers!