Hello Nils
My truck is a 46 and I have fiberglas rears and running boards with a repo Mar-K steel bed. I don't really don't know about the rear splash nor have heard about them but that don't mean
that they don't exist so I can't comment on those. I had my front end install and I matched the bed holes up to frame on 2 1/4 blocks then clamped the running boards into place and installed rear fenders into place approx. 7 1/4 " at the rear from bottom of bed. Everything lined up fairly well. Mine is still ripped apart after 16 years so maybe someone has a newer more recent explanation/experience. Have fun and good luck. Your son will remember.
Dan