driveshaft won't fit

[merlin3]

i am in the process of restoring a 72 c10 2wd,when i bought the truck it was lowered,had a 454 TH400,66 chevelle posi rear end.I replaced the trans with a new TH400 from Summit bolted that in same holes with a new mount,replaced the small block frame stands with the correct big block mounts,installed a 509 Merlin,i also lowered the truck with all new parts to were it was when i bought it,installed 5 inch lowering springs and 2.5 lowering blocks in the rear and kept the same rear end.the problem is now my driveshaft is too long,any ideas on what i did wrong?