[olddog]

The 260 has the same stroke as a 289, and a smaller bore. The 3.8" bore may well prevent you from using 289 heads or larger valves in the existing heads. With the small displacement, the factory heads may breath OK. I would port match the heads to the gaskets, same with the intake manifold. Do the same on the exhaust. Clean up any casting crap in the heads. Do a good valve job, and call the heads done.

If the block will easily convert to roller lifters, put a mild roller cam in it, and be done. I wouldn't try to go all out on this engine.

You might try reading up on what Shelby was doing with them in the AC Cobra. I do believe he raced a few of them.