I went with a 1500, a 1978 model. I love torque, and the 1500 is a good engine if you don't get too rev happy, limited to about 6000 RPM or so. I have always been of the opinion that there is no replacement for displacement. There are so many performance parts available for all the Spit engines, you are only limited by your imagination (and bank account!)
I went kind of crazy on durability & performance items, I even tracked down some NOS Vandervell bearings. Pricey, but worth it in the long haul. Double roller timing set, 9:1 pistons, full electronic programmable distributor, 3-2-1 header from Rimmer Brothers, Pierce fast road cam, lightened flat tappets, roller rockers from Ted Schumacher. Tuftrided rocker shaft and upper oiling kit, oversize valves, polished rod beams with aircraft steel rod bolts, port matched head and balanced rotating assembly, aluminum flywheel. Oh yes, and a Weber DCOE sidedraught carb!
Did I mention that I'm a gear head?