I recently built a 321 inch GMC (4.125 x 4) and used the #983 head and ported it. We got 264 cfm @ .600 lift. The exhaust sucks @ 144 cfm max. The engine made 347 HP @ 5500 rpm and 346 lb/ft TQ @ 4600. That's with short zoomie headers so I was giving away some TQ and HP. 4x1 header coming this winter.
I used 308 Hudson rods (8.115" C-C) vs 7.0" stock GMC rod.
1940-50's Chrysler, Dodge, Plymouth rod bearings
Pontiac pins (.984)
Custom JE pistons with a 50cc dome (11.5:1 CR)(very light)
3 Rochester carbs on a built manifold.
With just 4 runs under our belt the HAMB dragster ran 12.10 @ 120.5 MPH (New records for MPH & ET)
By the way the 270 and 302 have a 3/16" taller block than the other GMC engines.
I saved 1 1/4 POUNDS on each piston/rod/pin. The stock GMC piston and pins are HEAVY. The rod is strong but too short.
Ron
