fitment isssues motor trans drive shaft 71 swb

[tom71]

hello all i m having problems with my project i have a 71 swb truck and i put the motor, sbc and trans ,400 in today and the drive shaft seems short
the ,chassis is not the original ,not the one that the drive shaft came out of
i used the forward two motor stand holes on the chassis , i believed the rearward two are for a bbc the chassis is bagged and sitting on as low as it will go . so when i put the driveshaft in and try to bolt it to the diff it pulls completely out of the trans i believe it will hook up to the trans if i use the rear ward motor stand holes as well as the trans cross member holes but is this right? as i wrote i thought that the rear ward holes were for a bbc thanks for your imput , tom