I have designed and built two, I scaled down from existing lifts. In the end it would be so much cheaper to find one used and modify it.

Take a good look at the pics both show the cylinder pushing to the rear of the truck. Not shown is that the lift point is about 1/3 back from the front of the bed.

Biggest mistake is to under size the hinge at the rear. Next would having it lift to high, you don't want it to open much past 85* or less on a short wheelbase truck, especially on a truck with a small(light) motor. Danger of tipping the truck over backwards.

All the info you need is in the pics posted above.

Oh and if the frame on the dump bed is not ridged enough and you go to lift an uneven load (say very heavy in the left front and light in the right rear) then the dump box twists and the whole thing rips apart at the hinge at the rear, or at the connection of the lift to the frame rails. Then you end up with the load and the truck tipped over sideways.

Last bit O caution using the wrong cylinder (undersized) and the cylinder rod bends about half way up then comes crashing back putting a huge hole in the gas tank.

Last one (homemade) dump bed trailer I fixed netted me about $9000.00 they make great winter projects.

Good Luck be safe have fun.