http://www.oldchevytrucks.com/cgi-bin/htmlos.cgi/00657.2.6677316170015882229

The 1958-1962 261 truck engines had the larger holes for the the oil filter lines (1/2 NPT which looks a lot like 3/4 holes).

Most 261s had "captain bars" but this is not a "perfect" ID. Check the link above.

Tim