Gentlemen;
The early 'twelve ports' work well on any CID engine. The larger (GMC) the more benefit* as they 'breath' straight through.**
This guy has all the pieces and will probably list it as an 'assembly', in the $4,000 range around "tax time".
Good luck.
(*) The "stroker" 285/292 are limited to 5,000 RPM so there wouldn't a (cost effective) benefit there.
(**) The ('Sissel's) head probably does too, for 250/292 'later' engines, which rev much higher.
