bad vibration

high hows it going a little new to this. here is what i have a 1940 ford pickup with a 350/350 crate motor 8 inch rearend leaf springs in the back,coils in the front. everything on this truck is new. now the problem,at about 40 mph iget this pulsating vibration,i know this dosnt sound strange,but it dosnt do it in the first 3 or4 miles,only after that. my angles are set right im takeing my drive shaft tomorrow to see if it is balenced and right leangth. also nothing seems to be rubbing any where. need help it is driving up the wall!!!!!!!!!!!!!thanks for all input

"Pulsating vibration"--does it kinda go "vroom--smooth---vroom"? That's driveshaft, u-joints, or loose spline. If it makes the whole car bounce, or the steering wheel wobble, its tires. Try getting it vibrating, then shift to second and hold the same speed. If the vibe goes away, or changes frequency, its the engine, not the driveline.