Struggling under load

I am having a load problem with my slant six and need some help with diagnosis.

-Fresh valve adjustment
-Timing set to 10*
-No vacuum leaks
-Running temp idle set to 750 RPM
-Vacuum advance ran straight from dizzy to carb
-New plugs gapped correctly and sealing washers removed
-New cap and rotor
-New plug wires
-New fuel lines and clean tank with fresh gas

Now it will idle in park and neutral just fine but as soon as it is in a drive gear such as reverse, drive, etc. it bogs down terribly and picks up a rough idle.