A little background.  Have gone through 3 mattresses over the last 3 years trying to find one I can sleep comfortably on. I started with a memory foam that eventually killed my lower back.  Moved on to a sleep number which when set very firm, solved the lower back issue but I tossed and turned all night with the pain felt in the areas I was laying on(pressure points).  My last attempt was an adjustable bed, a split Cali King.  It came with a memory foam mattress that I suspected wouldn't work.  It didn't, as I sink way down. I know I need a firmer mattress and want to construct one out of foam.  It needs to be 8 inches High.  I was looking at Foamorder.com. They have a very firm conventional foam called V90 I would use as a base.  I called them and they suggested 3 inches of the V90(ild90), 3 inches of the V54(ild45) and 2 inches of some kind of comfort topper- possibly latex.  I am looking for feedback as I am getting really tired of spending lots of cash on beds.  I am 6'6, 290 lbs and sleep on my back or on my side.  Help!

If you are that tall and heavy I doubt 8 inches will be enough.  I think you'd need to go to 10 inches for additional support.  There are lots of people here who have done DIY foam projects, but I'm not one of them.  I'm sure the experts will step up !

I should think foamorder would know how much of a base you would need. However, to be safe, it wouldn't hurt to buy 5" instead of 3" for the base piece.
Do not use memory foam for your topper. Use a soft latex instead - maybe 18-20ILD. If foamorder doesn't have it, try foambymail.com.

I am assuming that V90 is an HR foam? If it's not, ask them if they have an HR foam (High resiliency). I'm also pretty sure that High Resiliency is high density. You may want to ask them about this. It is my view that HR is the best foam, the next option is HD. But I could be wrong on what those terms mean precisely. Eagle2 might know, he just read a lot of info on foam. Look at the thread he just posted about Foam Info and read some of that.

You may want to consider M-Grade foam at overnightmattress.com. They have an 8" base for a good price, with a sleep guarantee of 3-4 months in which you can return it for a refund. I do NOT recommend any of their other mattresses other than just the 8" base with no memory foam. Because all of their other mattresses have at least 2" of memory foam and I found their memory foam to be too soft. They have a basic model that is just 8" of M-Grade foam. What is M-Grade foam? They tell about it on their site but I have found no other source of info on it. However, I slept on it and I really liked the feel. As far as I'm concerned, the "warranties" on foam (and innner spring mattresses) are all b.s., anyway. No mattress is going to last 10 years. Maybe a latex one will, but not any other foam, imho, and certainly not an innerspring bed. So figure that an M-Grade or other HR foam may last 5-6 years if you're lucky. Foam breaks down. Period.

Thanks so much for your replies.  I checked with foam order and they have 3 grades of foam, the V90 and V54 are their highest grade.  The listed density for the V90 is 3.2/ft³  .  The listed density for the V54 is 2.9 lb/ft³  .
I am putting this mattress on the other side of a split Cali King so I have got to keep it the same height as the other side- 8 inches.  I will continue to educate myself with some of the other threads and I will look at the matress at overnight with the M foam.  Your help has got me started and any additional suggestions will be greatly appreciated.