[Firebirdjones]

Might be a low compression issue as part of the performance missing.

Those goodwrench 350's were only rated at 8.5:1 but I believe several magazines have torn them down for buildups and cc'd everything only to find it really only has about 7.8:1 with those large 76cc heads they come with, and the fact that the engine even has a dished piston in it last time I saw one apart.

It would help, and probably perform much better all around, if you went with a set of vortec heads and their smaller 64cc chamber, not to mention they flow decent for a stock head, and 170cc intake runners make great torque, better combustion chamber design to promote better burn, etc.... That would have been my first choice for a cheap engine type of build.

If those TBI heads also have a large 76cc combustion chamber,,,and you installed those heads with your typical .038-.041 compressed head gasket rather than the thin .015 steel gasket GM had on there, then the compression is going to be much lower than 7.8:1 that I've seen these engines reported to have.
When you couple that with a larger camshaft with more overlap that bleeds off cylinder pressure and kills your dynamic compression ratio, it really makes a lazy engine that feels down on power.

I'm just spit balling here because without all the specs and variables it's anybodies guess. Just some thoughts that crossed my mind.