Did you have any issues with the fit of the flooring once you added the 3/4 foam? When I did something similar on a 78 corvette, all of the molded carpet fit weird.
More/thicker mats offset the shifter hole markings when installing my new top mat. Maybe because the floor to footboard crease was filled in with about 3/4 inch padding and the crease wasn't as defined anymore and it effectively made the new mat about an inch too long front to back. No problem though, as the front edge perfectly overlaps the firewall cover and the back edge is well hidden by the seat bench.
I use the top factory style mat as long as possible till it starts getting torn and ripped. Then it becomes the old "bottom" layer and is replaced with a new top factory style mat.
Starting from the top going down I have:
1. Nice new floormat
2. old 3/8 felt with aluminum foil on one side
3. Old ripped floormat
4. adhesive sound deadening rolled on the whole floor and well into all the nooks and crannies.
No adhesive on top 3 layers; have always let gravity hold them down except this time I secured the new top mat under the door bottom edge weatherstrip retainers. Gotta have the master cylinder hole plug, otherwise incoming air tends to raise the mats.
