MSD boxes
I've used them on the track & the street and they always worked great for me - usually the idle is smoother and starts easier on "built" engines compared to single spark stock ignitions.
Often need to get their tach adapter too though, since many stock & cheaper aftermarket tachs...
I had a Campbell built race engine once
It was a CASCAR spec 355 s.b. chev originally built for Carl Harr & Daryl Harr and it worked great. Converted it to dry sump as soon as rules allowed & got lots of laps over several seasons out of it (with some freshenings) before we broke the crankshaft...
fender flares
Nascar would have had templates for fender flares, but the cars were very lowered & had no inner fender liner, so not sure how that would transfer to a streetable version.
That kind of sheet metal was usually formed in-house by the body building specialists for each team or...