As stated 34 degrees (max for a small block) is a tad low for a big block.

Did you check the plugs after you took it for a blast

Do you have a choke and is it opening?

Are you not getting enough fuel at higher RPM for some reason. Fuel filter restriction etc

You mentioned it was not as hot when you tuned it. The colder the air the denser the air charge charge, as you start to lean out you can make more power.