Most local body shops have the round head Chevy ones, not the flatter stainless covered /capped ones. The closest I found for my’68 was from Mr.G’s Fasteners, Ft. Worth, TX.
Not quite as flat as the original, but very close. Also has the bumper thickness flats.
I am not up to speed on the ‘70, but if I recall the R-T Specialties bolts for the other bodies were 5/16” whereas my ‘ 68 Polara used all 3/8” diameter bolts.
I got my bumper bolts from National Moparts in Beaverton Ontario.
They are the stainless ones with no caps.
But to each their own.
Once the bumpers were installed, The car as a whole was looked at more than dwelling on the bolts.