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I have to admit I've never laid the HobbyCraft kit over a set of plans, but most of the reviews of it in its various incarnations have been pretty favourable. Are the problems solvable?
Concerning P-36/H-75 with R-1830 engine. The fuselage from cockpit to tail is too high, I'm planning to use middle section of the fuselage from Monogram's P-40B.

The cowl a little undersized in diameter (max. 26 mm, should be at least 27.5 mm to cover R-1830)

Red - R-1830 dimensions.
All in all not too bad, but I'll correct it.
Mohawk with R-1820 is much worse

. They gave the same diameter of the cowl (26mm) as for R-1830, but R-1820 was larger (in 48-th about 3mm) add 1.5-2mm which they lost in R-1830, and you will have cowl undersized in 5(!)mm and accordingly undersized forward part of the fuselage. So for decent Mohawk one should make a whole new fuselage.
I'm working on the correction sets for both Hawks with R-1830 and R-1820 but I can't say when they will be ready

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Sergey.