I used a (pricey) Moroso adapter which was attached to the manifold with U-bolts. I had 2 pressure gauges at first to check out the pressure drop across the filter.
Spin on Filter for my 261.
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1948 3/4-Ton 5-Window Flatbed Chevrolet

34 Years. Now with a '61 261, 848 head, Rochester Monojet carb, SM420 4-speed, 4.10 rear, dual reservoir MC, Bendix up front, 235/85R16 tires, 12-volt w/alternator, electric wipers and a modern radio in the glove box.