I did that in my 59 Suburban almost 20 years ago and then again recently in my 58 2-ton. YES, an electric carving knife is the way to cut the foam (goodwill, Ebay, or CL). I used 4" of HRF-2953-C with an inch of HRF-2528-C on the seat bottom. I did no treplace springs on the back since they were fine and don't get your full weight on them. I used an old thermarest type camping pad (2" thick I think) to put over the seat back on my 58 instead of spending the 50$ for a roll of the cotton batting just for the one back.
