I think Mar-K stuff is pretty decent, so I would expect your box to be close to original dimensions / fit.
Fibreglass stuff is always a gamble. I tried a pair of rear fenders from Superior Fibreglass in Oregon - they were WAAAAAAY off on length, height, and fit to the running boards. When I questioned them on the phone, response was " oh well....the moulds stretch after a while and you have to expect some fitting " . To their credit, they took them back and gave me a full refund. I ended up with a pair of steel fenders that we could work with.
If you want, PM me your emai. I can take a few pics of front and rear fit of my original running boards / fenders if that helps.
