[mr48chev]

I think you are going to loose way too much height and legroom if you try to set the leanback back in that area and from this diagram you don't gain much if anything. The attached diagram has your stock seating measuremts and gives a pretty good idea of what you are dealing with.

I'd go along with the 88/ 94 or so split bench Silverado seat. I've got one in my 71 GMC and that is super comfortable after going from a worn out stock seat to some terrible buckets out of a van and then a plain bench a friend gave me out of a base line 88 4x4 That his wife had bought him a Flexsteel seat for. That seat was like a wood church pew an not comfortable even though it was free.

Another seat you might look at is late 90's and maybe early 20k Dodge Dakota. They are about a half inch wider than the stock seat in the 56 and should have thin backs.