Aaaah, what fun. Break out the tape measure, how much of the 1970 do you have? The unitbody construction is designed so things can be taken apart to repair, not sure what you have or don't have for the '70 to work with. Firewall forward should be the same, cowl and windshield is the same, and the A pillar should be the same, but then the roof to the rear window is different, including length of the doors and wheelbase. Your floor panels are different length, B pillar that the door attaches to in the middle is wrong for the '70 so that has to be changed, so you have a lot of work to do.
What might help with helping with the project is describing the condition of both cars, especially the '70, given it is the parts car. If you had the body and doors, rear windows and quarter panels, great, front on the '68 can be disconnected and added to the '70, rewelded (drill out the spot welds, that's how you restore with original parts).
Someone around here took the front of a sedan with the roof and doors and rockers, and grafted a coupe on from the B-pillar back, effectively shortening the wheelbase by the difference in length between a four-door-front-door and a two-door-door.