well sir, i did not mean to offend you in the least... i am just a uneducated hillbilly, but i am speaking from experience, my name is bobby walter and i am a detroit area native. i use my name because i dont have anything to hide. as i get bitched at, i find i do leave out details that i take for granted are obvious at times though.
i have assembled and helped assemble n/a 635 hp on the first pull, with out of the box bg carb, 302 windsor based engines, and feel it gives me no authority for anything in this topic.
i also under stand and demand for a stout long life build squaring and decking is a must along with top of the line balancing and basic blueprinting at the minimum....this in most places today is around 1000 to 1200 and thats if you live close to a shop you trust..and you better not be in too big of a hurry either

and we have not even put a parts list together

...just machine work...i have two tanks of gas to do that now if i want to build a engine where i live..
so doing parts selection and assembly my self on a standard stroke 302 base engine can easily run over 4 grand, and is a big reason i push people to look at modern low mile takeouts, for installation expenses i can swap in a modern 350 hp powertrain for what it costs to have good custom headers for a 302 swap made. and trust me sir, i understand when your customer demands x in y then thats what we are doing...
but with the x in y, and your specific demands you painted yourself into the box, but there are other options sir.
headers....i have used many sets. typically i would use a hooker, bassani, or mac type shorty fox swap header. "351 windsor fox chassis swap" . there are log style oem type headers as well from the sn95 and later bird setups. these are easy to modify as well.
they have a sn95 header application which requires a minimum of frame modding. some rolling on the passenger and driver sides in a tib or ttb chassis i would argue increases the strength of the chassis and will be necessary.
taking thier time with a torch it will increase strength imo and look factory after its cleaned up and painted...i have even done it without a torch but it increases stress risers.
back when i first started doing this it was mostly mid length headers and they required at least a 1/2 inch cut out of the frame rail per instructions. plus they were shitty thin leaky flat gasket headers. so i experimented from there on a poor boy budget....and back then there was no internet and information was much harder to find.
after busting my frame up and rewelding i was strict with proper radius cutting and then after a few years went to rolling the rails...(when possible)
the other thing is actual engine mounts used and actual engine location which is dictated by many factors, usually costs and looks.
what fits good with a c4/d20 or 1350 t case wont with a aod/ f208 tcase.
i have put 454's in a sla chassis with no steering mods...but obviousy custom fenderwell headers...and cooling.
cost of custom headers verse a lil frame rolling, look at your situation with a set of headers from summit or something for mockup and go from there.
worst case send them back
not sure how tight for time you are but this guy wants to make some too, maybe contact and work with him, i have no problems shipping him stuff to work with.
http://www.therangerstation.com/forums/showthread.php?p=1356519#post1356519