If you are on a very limited budget, and have to keep it more or less stock, then gearing and possibly a looser torque converter would be the easiest things to think about. As for the intigrated head, those are interesting if you really look at them, but not so much for peak performance. with the one barrel model, you could remove the sleeve under the carb, and open things up a bit, of you intend to add an adapter and larger carburetor. Also from observation some of these heads had smaller runners on one side of the plenum area than the opposite side. I have seen two and on them they were on opposite sides, so it wasn't something intentional, just sloppy foundry work. That said you can open them up to match them if desired, and also open them up around the boss for the bolt holes at the top end where the runners curve to feed the center cylinders. But on the exhaust side there is little room to open things up. Or you could look for a two barrel head in the junk yard. Those came with a dual exhaust manafold on them, which were joined further downstream. As for bigger valves, you would have to talk to a maichine shop to know what is possible. The same would apply to milling them. And of course there is a camshaft option too. With something around 3 to 1 for gears, don't go nuts here, you need the help at the bottom end.
Good Luck!
