I have an 87 d250 with a 4 speed manual. It's original engine was seized and had holes in the cylinders, so I swapped a new 360LA in it. the new engine came from a small motor home, it came with it's automatic tranny but no shifter or trans cooler. I swapped the old flywheel onto it, since it surprisingly looked fine, and put a pilot bushing in it. I had a little trouble getting the transmission in. (this was all happened while I had the new engine out of the truck) but I got it in eventually. skip to after I got it in the truck, I go to start it in reverse with the clutch pedal depressed, and it launches backwards and tries to take out my uncles house. I turn it off after I realized the brake and clutch were helping as much as my brother who was petrified.

After we got it all jacked up off the ground, we bumped the starter in all the gears with the clutch depressed and it was still moving. and if you start in neutral and try to put it in gear then it still don't work.

we watch the clutch under the truck and it seems to be working as it should. can someone please give me like a list of solutions

 

Clutch disc could be in backwards. A buddy of mine did that with his Chevy. If the slave cylinder is pushing the fork in and you can see it moving, most likely the clutch disc is backwards.

 

"I had a little trouble getting the transmission in."
What do you mean in the above?
If the trans hangs on the disc it's possible to bend the plate hub.
Did you indicate the bell housing in to the new engine?
What was your clearance between the input shaft and the installed bushing?
Did you set the plate departure correctly?
Some basics...…..
https://documents.holley.com/199r11317.pdf