I went to STD heads that have the standard STD 3 bolt flange, but I still use the Ultra High Temp Copper RTV.
If it's fitting tight but not sealing, I'd think the RTV would work better since there is no room for a shim.
I'd cut the slots in the pipe, smear some Ultra High Temp Copper RTV (or black) on there, clamp it down and let it sit overnight, then start it up and see if you have any leaks. Then you can get rid of the excess RTV that oozed out.
The Orange/Red never bothered me, I'm always able to trim the excess off, so it doesn't look gobbed up, just a nice clean Orange/red line around my exhaust. I prefer the Orange/red over the black, because if it does have a small leak in it, I can see the black exhaust on it, and re-seal it if need be