The front sub frame change was 73. Most header application pages wont include 75/up because of emissions and generally omit Dobas because not considered a performance vehicle. Just about anything spec'd for small block B-body will fit. A couple tubes on drivers side go under the tie rod, passenger side is a bit snug, some install instructions say floor shift only but there's usually room to get around column shift.

If you're going headers only because you can't find an off the shelf head pipe, find a exhaust shop with skill. Custom bent duals from manifolds to bumper can be had for $500ish plus mufflers. Been a few years since I had any bent, your prices may be subject to inflation. I always supply my own mufflers to get what I want and skip the shop mark up. Getting past the gas tank on the drivers side is tight, but again an exhaust shop with skill can do it.

FWIW: best money to spend on waking up this pile of steel is swapping the rear gear. Probably 90% of them shipped with a 2.45 that will give long legs and decent mpg on the highway but won't get out of it's own way. Ya won't need more than a 3.21 to have a completely new car. Add a swap meet cast iron 340 intake, a TQ and maybe a purple shaft (if they still make them) and it's bucks down, fun up.