I wouldn't put too much faith in factory HP ratings, they were typically under-rated at that time- measured at the back wheels, some with all accessories running, i.e. if the vehicle had air conditioning, they rated it with the A/C on.

I believe any rebuilt 351M with a stock-level replacement cam will pump out around 250 HP at the flywheel. With a 4-barrel and bigger cam and headers, 300 HP would be no problem. This is assuming no cat converter, EGR, or other power-robbing emission controls.