I have the same issue with my 1/2 ton '50 with a full pressure 235 and a dinky 216 clutch pack while pulling way too heavy of a load on a trailer. I found that gradual acceleration after letting out the clutch pedal will eliminate the slippage in all gears. If I goose it under load, it slips.
I did have the puny pressure plate resurfaced, which may have robed me of clamping force, as Pre '68 Dave suggested.
Carl

1952 5-window - return to "as built" condition | 1950 3100 with a 235 and a T-5 transmission