Just checked a 216, 235, and a 261 with the dowel pin. The return hole near the pan rail is in the same place on all of them- - -the same position as the small plugged port next to the 1/2" NPT lower hole on the 261. If you want to alter the flow on the engine in the picture to actually do some filtering, adapt the lower line down to fit the small port, plug the big lower hole, and add a Holley carburetor main jet drilled out to 1/16" diameter to the fitting that screws into the 1/8" NPT port. A 1/4-28 tap is close enough to the Holley jet thread to work- - - -I've installed bunches of them for people too lazy to clean out canister filter housings, who insisted on using a spin-on filter in a bypass application.
The 848 head on a 261 will give considerably higher compression than a stock 261 head, even higher than on a 235 because of the extra displacement. The 261's were designed as high-torque truck engines, and nobody wanted to use premium fuel in a working truck.
Jerry
