I'd keep the original engine and install a 3 speed Brownie. The 261 has enough power to keep the truck moving- - - -it just needs enough gears to keep the engine in its best torque band. With 12 speeds instead of 4, and an OD top gear to improve the road speed when running empty, you've got a lot more gear options with a lot less work and expense. There are usually a few available on Ebay, and some of the older truck salvage yards might have some stashed away.
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!