AN fittings are measured in 1/16" increments, so a #8 line is the equivalent of 1/2". That should be plenty of volume to support a full flow system. All a #10 line will do is contain a few ounces more oil, with no real advantage at the pressure and flow rate of the 261 oil system. Be sure the dowel pin in the flow control system is in the "in" position, as that's the only way a 261 will function as a full flow oil circuit.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!