Manual TCP rack & pinion
AOD trans

Recently installed some long tube  Flo-Tech's butt they hung-too-low. Got sum mid lengths butt ain't installed'em yet.
The long tubes went on pretty EZ because.........I wallered-out the bolt holes some on the headers AND when installing I RTV'ed the gaskets in place BEFORE I put the headers up....That way they stayed-put while fighting the fit-up.
I made a couple of short studs and screwed them in the engine first. Then.....I had something to kinda hang the headers on while I manuvered the other bolts in.
Some bolts were started with the flange out about 1/2". (understand?)  Put the bolts in the hole and THEN put the header in final position. Anti-seize is messy butt......sure makes the bolts twist in EZ.
Although I didn't need to.......sometimes the engine needs to be jacked up off the motor mounts a little.
A helper is a good thing too. Someone one to hold the header/collector end up a little while starting bolts. (Can help you cuss when necessary too)
May need to remove the export brace/Monte Carlo bar to help with install.
I like to run the engine after the headers are installed and burn off the header paint AND while hot give the bolts another torque also.
Next few times the engine is hot I like to tighten the bolts  again. When they won't turn anymore then I know they are tight. May help with your blown gasket situation.
I'm hoping these mid lengths will be pretty EZ(er) to put on.
6sally6

I installed my JBA shorty headers from the bottom. I took the Headman long tubes off from the bottom as well. My engine is a 351W with a C4 at the time that was done.

JBA shorties go in pretty easily but you might have to loosen the motor mount.  I have them on my 66 with Dart 427 and TCP rack.  They have the ball type collectors so no gasket needed there.
Gaskets at the head ports will last longer if a welded in H or X pipe is installed between the right and left pipes behind the headers.  JBA also makes a H pipe for those headers.  No gaskets used.