Daniel:

This topic seems to crop up once a year or so; I'm sure you'll get many opinions.  If you replace the bench seat cushion, you'll still want to use a very firm foam.  When you're laying down your weight is distributed, but sitting, it will compress a lightweight foam pretty easily--I learned the hard way. Jonce can probably give you recommendations for foam.  I purchased mine from an automotive shop, and was able to try various firmness level they had.

2nd, I purchased a 4 inch memory foam mattress.  This is basically made up of a firmer base foam with memory foam on the top.  This memory foam mattress goes in the rear, large cushion.  On top of all this, I use a 1 1/2 inch memory foam topper.

The nice thing about this setup is if you want a firmer mattress you can sleep with your head towards the front of the van, or if you prefer a softer mattress you can sleep with your head towards the rear of the van.  You can also go without the topper, but I find it even evens out the hills and valleys nicely.  I also put an extra piece of four-inch foam between the rear door and mattress, and this prevents "butt sag" between the front and rear cushion--hate that!

Lastly, I did away with the bolstered design of the seat and just left it flat, which to me is more comfortable for sleeping.  I don't have any recommendations for brands of toppers, but if I recall correctly I googled memory foam topper's and got more information than I could possibly ever want.  There are definitely different quality levels. I think I paid about $90.  I use one of those "space bags" to store it; it compresses quite nicely.

Cheers,

Troy