The Gen 5 block was an engineered disaster in terms of head-swaps using original Mark IV heads (not so much a problem with aftermarket heads) and engine-driven fuel pumps. A one-piece rear main seal, and the re-routed oil passages were actual improvements.
The Gen 5 ports and valves weren't any worse than the Mark IV "small port" "round-port" "peanut port" heads. The net-lash valve train was unwelcome. Standard cast aluminum valve covers were nice, though.
Early TBI 454s were based on the Mark IV--up to 1991(?)
The Gen 6 "Vortec" 454s used crappy swirl-port heads like the TBI small-blocks. Those heads are not the great leap forward that some magazines have implied. They were marginally better than the round-port heads they replaced.
The Gen 5 ports and valves weren't any worse than the Mark IV "small port" "round-port" "peanut port" heads. The net-lash valve train was unwelcome. Standard cast aluminum valve covers were nice, though.
Early TBI 454s were based on the Mark IV--up to 1991(?)
The Gen 6 "Vortec" 454s used crappy swirl-port heads like the TBI small-blocks. Those heads are not the great leap forward that some magazines have implied. They were marginally better than the round-port heads they replaced.