That amp should not require any mods to convert to 572B's.572B's are like 811A's on steroids.Electrically the same but with 160 watts plate dissapation instead of the 65 watts for an 811A.
I'm confused by a couple of your statments:
First you said:
Okay, I checked it out last night and here are my results, everything was done into a dummy load with a bird 43, with 10 watts carrier in on am
I got 40 watts out, plate current-75ma grid current-30ma.
Then you said:
increasing the drive to 25 watts got 60 watts out but the grid current won't go over 80ma and the plate current won't go over 35ma.
Are you sure that is right because more power output should result in more plate current,not less.Either way something is wrong with the bias because the plate meter should read around 110mA +/- 25 % (according to the manual) with the amp switched inline and with no drive applied.
Try reducing the loading control somewhat and see if that raises the grid current.It may be loaded too heavily.
Check R12 and R13 in the ALC circuit as well as the setting of R14 which is the ALC adjust pot.If it is set wrong it will limit the amount of drive from the radio regardless of what the radio is set at.That is of course if you have the ALC line connected to your driver radio.
After that :?: :?: