The G seems to be the most difficult since it was mostly only produced in mass during the war. There used to be a lot of surplus stuff out there but it's been a long time since 1945. There is a fellow on here that goes by Barnfind08 (Southside Obsolete). that has a lot of older NOS stuff and he may be able to help with the 226 and 254 engines. The H was used a lot during the late 40s and early 50s till the 215 came out in 1952. The M was mostly a truck motor so they may be a little bit harder to source specific stuff for them. They are very similar to the H series and some things are interchangeable.
Ford produced a lot of information after the war. The best prewar information comes from the books published by Ford of Canada. The parts catalogs are reproduced but it's hard to beat originals. The Military G8T TMs may be good for the G-series engines and also the GTB 622 Burma Jeep TMs.
California Bill's little book on Hot Rodding V8's had a chapter on the sixes and the M engine. Reproduced with a white cover and red lettering. Sold all my six stuff and a good M engine [49-51 f6 and up trucks] Newc