You won't recoup the cost of repowering, that's for certain. What percentage, I really don't know, but definitely less that half.
I would suggest you take a look at repowering with a proper marine-built 383 stroker, much less weight than your big-blocks and a bit cheaper too.
Give Michigan Motorz a call,
www.michiganmotorz.com, they will be happy to discuss various options with you.
