Are number matching or original components a big deal?

First and foremost the car belongs to you. Personally I'm not a numbers guy. I know what it can do to value, and can appreciate their presence. But honestly I don't own to collect and build value - I own to enjoy. In terms of numbers matching the value has a ton to do with what we're talking about. A fast back A body Barracuda is a fairly hot car now, but a '67 273 IMO is not a performance ride like a Formula S, or a 4bbl car. Cool ride - but value is not tied to numbers matching powertrain like a Formula S car would be.
In regard to the questions:
" 1. Should I just save original 273 and A904 for the next owner or can I just pull the forged crank from the 273 and hold the 920 closed chambered heads and let the rest go? If you have the room, save it. If not, sell it. No reason to keep the crank or heads IMO.
2. Will it hurt the value if I add headrests (for safety reasons) to the 67 buckets? It wouldn't bother me. I probably wouldn't even notice.
3. Besides the fact it cost more to find and build a 340 vs 360 but will the vehicle have more value and buyer appeal with a 340? Not to me. 340s were not in '67s, and a 360 looks the part with potentially more streetable power and lower overall cost to get in there running. If you can score a 340, cool. But I'd go 360.
4. Do stroked engines turn on or turn off "most" buyers? All depends on the buyers. If the car was a year the 340 came in, many might want it left alone. Someone like me would appreciate anything if it's done well and the car performs. It's a '67 and you're changing it anyway. Keep it on the milder side if you want to keep the buyer market bigger and I think the majority of potential buyers will feel good about it when they drive it. If the brakes suck, and the car rattles your teeth, and you can't hear the guy sitting 2' away from you, your buyer will need to not care about any of that.

I'll add - Mopars have body color engine bays and under hood areas. I take HUGE points off for blacked out. It could have a 426 Hemi in it, if it's black under hood and gold everywhere else, it's a poor, half-assed attempt.