if fenderless,,,then just extend the firewall 6-8 inches is best option if you want to sit reclined and comfortable for long drives.
but if it is a local burger joint sitter, then don't spend the 10 hours to do it,, just sit like a librarian.
you can compare 50s cabs vs. 70s,, and you will find 8 inches longer cab on average.
40s-50s extended cab mods to the "cab b-post" rarely turn out proportional.
the "number one" reason people sell their rat build is --NOT COMFORTABLE--
more than a 20 mile drive.
a bummer after year to build and money spent... but it looks cool..
remember this is a rat rod ,,think outside the box,,,be creative site....
thats my opinion