I just did swap this weekend with full leather from 94 park ave. i had 60/40 cloth
On the rear leg you have in pic...the oblong hole I notched for stud to fit.
The bench in back was flat where as OEM had hump I flipped seat over, removed some staples, (cutting wheel)with some steel backing plate so i wouldnt cut through leather etc and I cut the frame and then i took a peice of 3/8 stock i had, bent it to half moon using hump from oem seat as templet, removed some foam and tack welded it in place of the frame i removed, and as Chiltons says...to reinstall reverse procedure.
I did have issue with my new front seatbelt clasps as they didnt want to lock onto oem shoulder clasp so I pulled the female ones from back seat, unbolted the Park ave ones that were attached to bench and replaced with ones from my back seat and no one will be riding back there so i didnt need belts,
But for some of you guys with the whip set up you may want to keep em cause your pasenger might start off in the front but end up in the back....but im not there yet.
I just removed my oem long female ones from tunnel and i did that due to the one has the extra belt for middle pass and i didnt want to just cut it off and i couldnt run em still attached to tunnel with new electric 10 way mutli adjustaple pimpy fine leather.
my new power seat controls are now located inside my consol armrest
NOTE: I just notched frame, welded stock then cut frame through just in case...didnt want frame to go BOING