+1 to the VERY sound advice from Charles. I wish I would have done ours that way a few years ago. Once I had committed to using the TMI foam, I couldn't reverse my demolition of the old foam from the seat frame. I've done our front buckets twice ('66 Red Standard). Used Distinctive Industries covers over original foam the first time, then used TMI covers over their foam the second. The passenger side (because of the "poofiness" to the foam, just never came out the way it should have, as I have a slight wrinkle in the bottom cushion. No one notices it or says anything but I sure see it....one of these days I guess.
I will also add that the new foam will give you a good workout, getting those covers to stretch over the (seemingly or real) extra size. This was when I was doing the job in warm weather, outside, living in Southern California. Believe I said "darn it" several times, as I blessedly recall...
-Lance