A Powerglide has two gear ratios- - - -too low and too high, plus the cast iron version needs a set of overload springs just to haul the thing around. The later aluminum case Powerglide makes a good drag race or dirt track transmission if it's beefed up sufficiently to take the horsepower of a full house engine. I don't think the 60 Impala trans would be a good option for you- - - -just swap the bellhousing and flywheel and stick with the standard trans, or adapt the engine to a later-design transmission like a TH 350 or a 700R4 if you want to be a shiftless trucker!
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!