Line Lock with Drum Brakes ?

Been there done this…lol. First, primary shoes on both sides, front and rear of the brake assemblies.

Second, brake pressure gauge and figure out by pumping the brakes prior to line lock engagement what works best.

Or…in lieu of the second above, adjust the proportioning valve.

This is of course assuming that all brake components are fresh and functioning as intended.

You can also consider a different master cylinder for increased piston area.