It sounds like you have adequate shoes for the load you have in mind, is your hydrovac working properly? Modern Chevy and GMC 3/4 or 1 ton trucks and Toyota trucks have proportioning valves hooked to the rear axle with a simple rod so when there is a large load, more power is sent to the rear brakes. One could get one of those valves and hook it up as designed, or
Make you own connection. Your truck might not have enough differential height to work properly as designer for modern trucks.

Ed

'37 GMC T-18 w/ DD 4-53T, RTO-610, 6231 aux., '95 GMC running gear, full disc brakes, power steering, 22.5 wheels and tires.
'47 GMC 1 ton w/ 302, NP-540, 4wd, full width Blazer front axle.
'54 GMC 630 w/ 503 gasser, 5 speed, ex fire truck, shortened WB 4', install 8' bed.
'55 GMC 370 w/270, 420 4 speed, grain, dump bed truck from ND. Works OK.