Jeff,
Recieved the headers and had a look.
They do not seem to be ground, but on the back side (casted letters) they say "Chevy" .. On the front side there are stamped letters "gmc"
The overall casting seems ok, there are differences in thickness here and there. (acceptable)
Issues so far: the front header do not line up with the exhaust ports on the head, either I'm crazy or the distance of the header ports are about 3/8' to narrow/short.
I think it will be ok with alittle creative grinding on the inside of the header ports..
I am using a stock 2bbl nos military manifold on a small port 270, so i need to use port size adapters, fabricate most of the clamps, Fabricate heat plate etc.. But that's general hot rod headache
Overall impression of the chevys of the 40's Fentons for GMC: Ok, not superb but ok.
