Why take a giant leap backwards in performance from the 261? The 250 will fit the V8 front motor mount position for the 59 frame- - - -just use the OEM V8 bellhousing and starter for the conversion. You're going to miss the torque of the 261 since the 250 is designed for higher RPM operation and has a shorter stroke. Swapping to a 292 instead of a 250 would take about the same amount of effort, and that would make a lot more sense.
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!