You were definetly too lean with them 66's on the primaries.
When you added that single plane manifold you killed a ton of bottom end torque but gained alot of upper end HP, jetting it up the primaries should help out the stumble.
If it were me and I wasn't runnning a P/V I'd jet it 80 sqaure as a starting point, then after making a full pass I would read the plugs and jet up or down as needed.
let us know how it pans out.
Cp