Has anyone tried using an additional exhaust manifold header flange to gain spark plug clearance?

Not for spark plug clearance, but for exhaust manifold clearance on eddy heads. SB A body. A header flange and longer studs. Worked for me.

What heads and headers are you running Larry ? I'm running Doug's D452 headers and Indy EZ's with Firecore wires and zero issues.
Gus

On my SB, I use flange adapters to allow std flange headers to work with my W2 pattern: it helps with the 5/8-hex plugs, and also helps clear the booots (Taylor). Doesn't allow 13/16-hex, they still interfere slightly, although less than without the adapters.
On a BB, they might help depending on their shape, which heads & angles you have.

I have. I welded another header flange to the header to move the tubes away from the cross bolts on an Indy block.

Running RPM heads with older style Hooker tuned headers. Current number is 5209. I wasted money purchasing the Accel shorty plugs. They are less than 3/16" shorter. Didn't help at all!!! These headers have separate flanges. I actually have to loosen the #7& #8 flanges to get to the plugs. I wanted to go with the Doug's but they are not a tuned header! TTI was also an option, but I would have had to buy a $300 starter, plus the $700+ for the headers to make them work. Too much $ for my blood. Got what I thought was a good price on a nice set of coated Hookers, but now have issues. May put my old CPPA's back on, even though they aren't a tuned header.

Do TTI headers need a specific mini starter or can you use a Dakota 318 2 wheel drive starter? That's the starter I run. It's the size of a mini and costs $80ish

The 2" tube and larger TTI's need a special starter due to clearance issues. The 1 7/8" and smaller do not. The stock MOPAR mini will work. Mind you, I'm talking a manual transmission with a 143T flywheel. Automatic transmissions are another story....