The only 150 of the era that weighed only 300 Lbs was the Merc inline 6, all the others were V6's and weighed in closer to 400 lbs.
The power / torque difference of a prop rated 140 vs a flywheel rated 150 wouldn't make me worry at all especially on a 21'r, my 18' SS is rated at 140 and my 22'...