Sounds like you're gonna be  removing & grinding this weekend!!
(Welcome to the best forum on the net BTW)
You DO realize these things are never really "done", don't you?!
Looks to me like the flange material is plenty thick enough for you to remove some of it to give enough clearance  to change/pull the plugs.( You would "think" the manufacturer. would trial fit its product and check plug accessability,huh? )
On second look.........if you grind out space for plug access...it will remove one of the bolt holes (or not!) It will be close.
Thats not even considering the hassle of moving the steering to get the header out!
WOW!!
Maybe mark each spot really well and THEN pull the headers and do the work. Maybe with a plasma cutter!?!
Some of the 4 tube headers fit without removing any more than maybe the Z-bar and/or the starter.
I'm a big fan of the "sound" 4-tube headers produce but.......hi-po manifolds are suppose to be wayyyy easier (for plug changing) ...don't cost much/or any HP at low speeds and  the Hi-Po 289 Mustangs had'em.(can't believe I would reccommend  NOT using headers)
Your call....I guess.
Let use know what happens.
Maybe some pictures?!
6sally6

Welcome to the site. It’s a great group of folks.

I have the FPA’s in mine and I like them. Buying a second set of headers may not be the best answer.

Turnng down the socket is the most economical. Just don’t forget to take it with you on trips.

The better option while less expensive will be a great deal of work is to pull the motor and grind/test fit where access is easy. Spending some time now may save a lot of time and aggravation later.

I did a test fit of the headers and Borgeson box in my car before it went to the body shop. Had to rent a motor hoist to do it. Used a bare block and single head with the bell housing and trans installed. It was time well spent. Didn’t think about plugs and headers thoug, got lucky on that one.

Can’t tell you how many times I did something over on my car cause it didn’t work out like I wanted or it interfered with something else.

Yes, welcome to the best place on the web. 

And remember BB's rule:  Any modification begets at least six other modifications which will in turn, each beget six more modifications.....and on....and on.

As 6S6 noted, the Hi-PO manifolds work decent at lower rpm and are way easier to deal with.  Butt (TS&T), of course, the cool factor is not there.  Also, when I installed the Hi-Po's on my AFR 165 heads I discovered some fitment issues.  Don't know if the 205's will have the same problem but be advised.  My problem involved making the manifolds set flush against the head.  This created gasket crush problems and involved quite a bit of material removal on the manifolds and some on the heads.

REMEMBER BB'S RULE!!

BB

As much as I hate using them, you might be able to fit a die grinder bit in between the ceramic and the flange and work around the ceramic. I would put in an old spark plug first. Might get enough to get the socket on.

Just remembered....I have a spark plug socket with a hex head made into the top of it. m(pretty common, I think)...Anywho that should solve your prob unless the header is just made worng?!!
6sal6
PS..at least it gives you a bump-to-the-top!

Thanks for the welcome and all the ideas.   I like the socket turning down idea, but my dad said we will still have the problem of the flange melting the boot.   It looks like we will be grinding soon.  I've already pulled the tops off.  We're going to see if we can do it without removing them from the car.

I use the thermo sleeves TKO listed for my plugs and the speedo cable.