Both Finn and Qweqwe are correct.
The P-39D manual states the original Take Off/Military boost which is 42" Hg at 3000 rpm. At this setting the Allison V-1710-35 engine has a power output of 1050-1190 hp, depending on altitude. The 305-315 mph speeds are the data we can find from real life trials, and our P-39D flightmodel's top speed is 308 mph at sea level, at 42" Hg, 3000 rpm. This is quite in line with the ww2 data.
Qweqwe's Grandfather was also correct about 350 mph at sea level. This since the pilots soon found out that the Allison V-1710-35 engine could be pushed to 56" Hg, 3000 rpm for WEP without damage to the engine, while engineers at the start had limited it's usage to 42" Hg. At 56" Hg the engine had a power output of a whooping 1470-1520 hp, depending on altitude. The dragco of the P-39D flightmodel is set to match 308 mph at sea level at 42" Hg, and this dragco along with the power output at 56" Hg gives our P-39D flightmodel a top speed of 348 mph at sea level, which is quite on par with the 350 mph top speed that Qweqwe's grandfather stated.
The same goes for the P-40E and Mustang Mk.I, which both are using the Allison V-1710-39 engine. Their pilots also soon found out that the rugged Allison engine could be pushed to 56" Hg boost for 5 minutes of WEP, which was much higher than the original factory recommendations.
<S> and I hope this clears it up