The 261 uses the old stovebolt bellhousing pattern (shared with the Chevy 216, 235 and GMC 228,248,270 et al). as opposed to the 250s SBC V8 pattern. The 261 is very desirable to those of us with pre-1963 trucks, so you should have no problem arranging a trade for a 250. Unfortunately, I don't have one ;-)