If a guy is looking at parts made by AMD, or equivalent reproduction pieces by another company, what might be a fair ratio/ multiplier to figure what a bodyshop would charge to install/ paint them.
I mean repair / replacement along the lines of rusted quarters, rocker areas, inner wheelhouses, hood hinge area, etc. Spots that are going to take welding and finishing skills.
Not "bolt-on" type pieces like valances/ hood/ decklid/ trim.

I know this is a very open ended question with lots of variables, so just hoping for a "rule of thumb # $$$".

I'm am considering getting a '72 E-body where i know the return value compared to investment will never be there should i need to sell in the future . It's a car that belongs to family/uncle, i cruised in it as a teen passenger decades ago, praticed in it for driving test, etc.
Essentially its the car that led me into owning probably 20 older mopars. But fuzzy warm feelings aside, i need to have an idea if the bodywork costs will crush the budget i could allot to it, or if i could afford to do anything else fun simultaneously with rehabbing the car. I am used to buying solid, basically no rust cars, so i want to be informed 1st.

72 Challenger needs, from my quick inspection, [ it is across the country ] no pictures sorry. Needs rear quarters/ maybe patch panels around rear wheels- swiss cheese, needs rockers-rusted, front fenders behind wheels- rusted, hood hinge area described as rusted, trunk area probable repacement needed also- described as "surface rust". Did not see in the engine compartment, so am unsure if the hood hinge area can rust without affecting the firewall or cowling {?}.
Did not get under car, car has sat in garages for last 15 years, so i think the suspension points/torsion bar areas are solid - but thats an assumption. Interior floors felt solid from the top, and no obvious water leaks into the car. OK, Thats all i've got, until i ask for specific pictures. Thanks.