when running minimal or no bodylift like you guys, thats the one place the early mustang/comet type headers can get ya. my floors were HOT. run long enough in a oem floors condition it cracks the floor and or burns up the sound proofer in a nasty way depending on power and tune...
one thing i have learned over the years, most of the underslung headers (#1 and #4 tucked) will accommodate this swap though.
but, for me......the dump on the sn95 and mn12 style manifolds on the passenger side is worth the frame mods from a workable exhaust perspective. its a preference thing so no rules here to be sure.
obviously with the previous clearancing from the mid/long tubes from years prior are skewing things a bit, but it puts the dump in the exhaust in a much better position to keep the floors cooler....a simple zip cut to an existing hole and a few minutes with a 15 in crescent wrench to roll the rail down slightly makes for all the clearance necessary.
the drivers side is only done for frame flex compensation...and i regularly destroyed the engine mounts so the more room the better. fawking thing twists up like a slinky. or at least back then it did...i have custom supports and the flat bed now that stabilized the frame a bit. but most of the early swaps i worked on in the late 80's early 90's had lots of issues with rub and leaky flat gasket headers....often times it was from the amount of flex that flat plate style mounting on stock 2.9 mounts....lot of years to learn the little things overall.
the early mustang/comet style 88400 dumps i would wager are far and away the easiest most cost effective setup at this time. really they have been all along.
flowtech makes some with larger primaries amongst others so you can put out a 400 hp 351 pretty painlessly if you want to. just depends on how you choose to mount the engine as to what you can get away with...of course stock ttb frame can cause other issues for ttb/tib guys but those with custom a arm or sas front suspension, things are opened up and much better for choices.
in your case with a classic style setup....the 88400 may be the best solution.