As you ask, ask about multi-angle valve jobs and if they can open up under the seats with a Serdi machine. Those are lower cost steps to get a bit more flow (versus real porting work). The Serdi machines are not everywhere, and I don't know if there are limitations on when they can be used, so that may not work out.
See if you can run down an old school machine shop in your area that does a lot for track work; IMHO, that will be your best bet. And, the shop that did Bob Glidden's heads back in the day is around Indy somewhere; dunno the name, dunno if the older guys are still there, and the price may be waaaay up there, but it might be worth running down. Are you anywhere near the Toledo area?