Are you sure this truck had power before you rebuilt the engine if not l would be looking at possible a bad torque converter see if you can check stall speed.don't know what truck you are working on but can get spec from dealer on trans stall speed.

Andre,

Will the lift move at all? Make sure the transmission fluid is at the correct level. When you removed the engine to rebuild, did you make sure the torque converter was bolted back to the flywheel? Not trying to pick, but I have done some stupid things in my day. If you jack up the drive axle and release the brake will the tires turn? Thanks.

We checked everything and the timing is 100% but the power is not transfered to the gearbox and wheels.

We check everything and the timing is 100%.
When in neutral it revs up, but no power is transfered to the gearbox and wheels.

Thanks Scott, will check accordingly.

Thanks Fishmech, I will check it out.

Andre,

Scott is correct. Check the timing first. The FE engine is very simple. Pull the top front plastic cover off the Cam gear. Turn the engine over by hand with battery disconnected. This will prevent any chance the engine will start. Find either the letter A or the number one on the front of the timing gear and align with the top notch in the front aluminum cover. Now check the engine pulley and make sure the mark is aligned with TDC, top dead center mark. You can remove the number one plug and put a screwdriver in the cylinder. Turn the crankshaft and make sure the number one piston is at the top of its stroke. Remove the distributor cap and make sure the rotor is pointing at number one spark plug wire. If the timing is off at any point, start there first. Good Luck

First did it go slow before you rebuilt the engine and if not l would check does the engine race up in neutral if it lacks power in neutral l would check your timing it is possible that when you put the distuburtor in you are off a gear tooth when you dropped it in. Check it with a timing light make sure you block the vacuum advance line.