OK,I'm no pro engine builder...... the head has Chevy small block V-8 valves,1.94 intake and 1.60 exhaust.They are Manley Street flow,7 bucks each.The exhaust seats were replaced,the intakes opened up slightly ,standard hot rod stuff,any decent machine shop can do the work.The springs are Crower singles with a damper, so called Z-28 springs.To get an installed spring height of 1.700,a set of Manley lower spring cups fit right in the stock valve spring recess in the head.The slightly shorter valve stems messed up the rockerarm geometry,simple fix on a GMC,just drive down the valve contact pads,weld them in the correct position.
I got a cam reground by Colt Cams in BC Canada to my own specs,they do excellent work.Delta cams in Washington can do it as can Lazer Cams in Tenn or just about any of 100's of cam grinders if you know what ya want..The typical guys who have ready ground cams for these engines,Langdon,Clifford,Howard's,base their total valve lift on a rockerarm ratio of 1.5,but actually as measured,it's 1.4. So right off the shelf, you're loosing .020-.040 total lift and some duration.
I bought header flanges on Ebay,the guy has some Chevy 235 flanges advertised at the moment.The flange port openings were too large, probably for some wildly ported engine.It was a pain in the [censored] to weld in material,better to use 235 flanges and rework them to fit. Or just get a pattern from a stock manifold fange and have a fabrication shop do a set for you.At least the flanges are 3/8 stock and cheap at around 35 bucks.I got got some 1-5/8 and 1-3/4 U bends from Summit racing,cut em up,welded it all together,a 10 inch steel cutoff saw blade on a chop saw or radial arm or table saw make it easy to do square cuts or angles.I made no effort to "tune" the individual pipes as it doesn't matter on a relatively low RPM street engine.The 4 bbl intake came from Patrick's,1/2 the bucks of Clifford,using a 390 Holley 4bbl for starters.
The oil pump pick up tube is fairly small on GMC's,a reworked larger diameter Chevy small block tube with a home made pick up looks better.
This engine wasn't cheap,I do my own assembly and design,but all the machine work,Venolia pistons,valve train,a million little this and that's has cost around 3500 bucks,ouch!Even the bearings were a fight,the only readily available mains are made in Mexico Federal Moguls,took some searching to find premium Clevite 77's.
When I started building this engine, I got a lot of bogus information from from vendors and guys who supposedly are GMC experts,like a GMC is some special magicial engine.
