Re: 260 cylinder heads?
Been so long since I messed with the 221/260;s, the 260 heads had fairly small chambers, valves, check the size of the larger, 289's may throw your compression ratio way to low.
I seem to remember the later heads valves may not clear the little 260's cylinders, but someone will chime in that has better memory then I.
I added a 4 barrel intake, 600 holley & duals to my first 260, the results? Terrible gas mileage with very little apparent HP gains.
(don't even think about replacing the one head with a 289;s, have to do in pairs, for sure,) but I don't think it would be worthwhile, as far as any HP gains, if that's what your looking for, grab a 289/302 & build from there