raised bed floor question bagged truck

[Beatrice]

i'm getting to the point of mounting my new fleetside bed on my 63'. this is my first c10 build so i was wondering from anyone thats raised their wood bed floor how many cross sills are you using? my truck is c-notched and bagged so i wasn't sure if i could just use the front two and just the rear sill.

I looked around on here but most pictures are down and didn't see much info on my specific question