The 221 and 260 engines have very few differences compared to 289 or even 302 engines. Most parts will swap between all 4. The 221 has very small combustion chambers and small valves, the smallest bore sized block, and a breathertube/filler cap in the timing cover. All other components such as bearings, timing chains, heads, manifolds, timing covers etc should swap. I believe the 221, 260 and 289 all had the same crank also.
Remember that if you go to rollers, you'll need the type made to retrofit the pre-roller lifter engines with the rollers (there are several companies that make these). The 5.0 (roller lifter) blocks have taller lifter bores and the valley has bosses that are drilled and tapped for "spiders" that locate the rollers and keep them from rotating. No reason not to use rollers, just be sure to get the retrofit ones. I can't swear that they work in the little bitty block, but I'd be surprised if they didn't.
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