I read thru the whole thing, then scanned looking for a couple things.
You've been asked time and again about engine location, you always deflect saying it near perfect and later state it's 1/8 in from center.
Do you realize these motors are set 1 1/4 in. to the passenger side (see attached).
That's almost the depth of the disfigurement you did to the tubes, dontcha think ?
Also, mentioned several times is the driver mount being odd !
If you look at all other mounts shown thru out the thread, all drivers side mounts appear to use to top hole at of the mount bracket.
That hole is the one needing modification spacer to convert the 273/318 mount to 340/360 use.
Your mounts don't use that hole at all.
I believe the initial not finding the bolts, indicates something.
So if your motor crank is in fact 1/8 inch from center, as one post states, and the mounts are from a van or pick up, NOT an A body, then the rant should be about great headers, shitty installer.
JMO
Footnote #39
Like many others here, I have installed dozens and dozens of headers thru the decades,
4 sets of TTI, and about to install 7th set of Doug's in the last 10 yrs, and never experienced the grief you are having.
Interference occasionally, - sure, mods needed that are well documented, mods needed not documented, lol , and experience..
It is to be expected that novice installers have difficulty, but reading intruction can help.
Put the engine where it's supposed to be with proper mount brackets, and insulators,
Good luck !!
PS. You do know the whole column can be moved at the firewall by loosening up 3 bolts on the floor.
Mosta of the previous advice given is absolutely right on.