more info on casting #s and changes....
as far as manifolds....
all 4.3 blocks use the same exhaust manifolds, but the intakes are different for the pre Vortec (12 bolt) vs the Vortec (8 bolt). The change over happened about 1996 or so.
if you have an older OMC 4.3 with the one piece exhaust manifolds (1985-1990) and need to replace them, since they are NLA, there are a few options, I think GLM had a kit to adapt their 2 piece manifold/elbow units to the original OMC Y pipe. Since OMC changed to the 2 piece system in '91 I think (and same parts were used by Volvo Penta from '94 all the way to the cat converter models) you can:
Buy a set of Barr Marine aftermarket manifolds/elbows
Buy the needed hoses and 90* down pipes from V/P (OMC and V/P used the same exact parts but OMC doesn't sell them any longer) and it will fit just like factory. I did this conversion on my 1988 4.3 a few years ago.
I went this route instead of the GLM kit because I had heard of problems with the GLM stuff, but it could have improved over the years. The GLM kit is cheaper than using the Barr Marine kit + the V/P parts.
1985-1990 one piece exhaust manifolds
Later style 2 piece exhaust (Barr Marine manifolds/elbows & V/P hoses and down pipes)
