[rotorwrench]

I think all the V8 mustangs of the 1st & 2nd generation were built with the same structural reinforcements in the torque boxes but they varied by year. The heavy duty suspension was also required for GTs. The 390 likely had its own front springs for those packages. The high performance 289 package likely had similar springs to the stock V8 cars but used with the heavy duty front end package that was a part of all the GT cars.

Most of the factory stuff was still not quite up to par for racing. Most folks added their own beef up parts or they repaired stuff as the wore it out or broke it. The old 390 Mustangs could break some parts.

The 351 Windsor had a really nice set of main bearings for a small block. It made for a heavier crankshaft and block. Everything was a bit beefier than the smaller engines even though some parts will interchange. The 351 C engine is almost as heavy as the 390 engine. The early 1971 400 may have been a bit heavier than the 390 in some applications but they were pretty close. Those two engines were a pretty close match too but the 400 changed not long after and became a smog dog.