ok so the 310 hp L29 is essentially a truck system. were the 385 mag or 500-525 efi's used on any vehicle with tires instead of a keel?

no

Only by the off road crowd looking to be rid of float bowls in big inch crawlers. The 496 intakes appear to be converted truck units , the 7.4 vortec truck intakes i believe are cast from other metals and the rectangular port mag intake might of found a home if they reserected the 70 chevelle . But it would of looked funny.
and the 525 efi asm unit might have clearance issues in a car. But would have looked sweet

I saw one time on Pawn Stars a guy put an offshore race motor in his drag / street rod car. Said he wanted the car bullet proof or he was trying to make a sale.

ZR 1 Vette, 32v, V8 built in Stillwater at the Merc plant mid early 90's, round ’94.

i am looking at this 75 vette. 350 q-jet. only thing interesting in the engine compartment is the air filter seals to the bottom of hood scoop. 290 hp smog engine. BORING!!! be interesting to open hood and have people say "nice setup. where the hell it come from?" i dunno if it would fit though.

You mean 190hp? I don't think any smog 350 was near the 300 horse mark. I drove a 76 or 77 a while back and lets just say most modern 4x4 pickup trucks would take it pretty easy.

No as far as big blocks, but this is probably the closest bbc efi (not motor) configuration:

Looks like the 385 mag intake might fit under a vette hood. That ZL 1 never will... and yes i guess it is a 190 hp. Tamest vette since the 54 blue flame six.

A vortec 383 with the early mag intake (mpi but with a carb flange a throttle body attaches to) with a proper 6000 rpm cam and head mods ,would be responsive reliable and easy to maintain. Homeless guy under the bridge could put together a turbo 350 or 700r4 to put behind it . Wouldnt be terribly expensive and would make quite a sleeper. I wouldnt want anything to do with a 73 up big block c3 personally