Issue solved
Sounds like you have a decent build - what cam gear are you using? What is the PN on the cam gear?
Your problem sounds similiar to my mistake - I put my LS2/3 build back together and used the wrong cam gear in it.
Car would start then **** the bed. I had to pull everything back apart and put in the correct cam gear - then it ran fine.
It was tge same one used with tge crank by the previous owner of the short block.

