I've had a couple people ask about this build so I'm going to post a VERY abbreviated thread to bring you up to speed.
I bought this 1968 SAAB from John (80' 427 here) about a year ago.
I then bought a '94 Miata for the drivetrain.
In two days it looked like this.
Drug the Miata hulk out and drug in the SAAB
Cut out the floor and firewall
New floor going in. Firewall moved back about a foot.
Cut out engine bay.
Welded in Miata firewall and subframe mounting points.
Subframe bolted in.
Firewall welded in.
Started on rear subframe mounting location. It also holds the gas tank,
Cut out place for rear subframe mounting.