need help engine doesnt fit

[mrgoodwrench_420]

i have a 68 gmc pu 1500 and it came with a 250 and a 3 spd i am putting a 350 and a 5 speed in it i used the bellhousing from the 3 speed and put it on the 5speed so i could use the mounting bolts on the bellhousing and when i try to put my350 in the valve covers hit the fire wall it is almost like the trans sits back to far also i am using mouor mounts and brackets from a 80 gmc and was going to see where the engine sat and drill out the holes but trans is sitting back to far what do i need