IMHO, and based on years of experience trying the cool looking stuff, the best bet would be a single 4-V carb. I've tried the dual-quad and tri-power setups, but surprisingly the only ones that seem to work the way I think they should are the early (61-63) Ford 390 tri-power setups. I found the dual-quad setups tricky....progressive linkage helped there (and was "mandatory" on the tri-power setups).
The thought of trying to synchronize 8 webers after trying to sync the 4 carbs on my Honda CB500 motorcycle just dumfounds me.....too much work and WAAAY low on the "per dollar" value considering how little power the webers add. There are plenty of people out there who like the linear power delivery of the webers, but they are picky about cam selection and they take a lot of fettling.
A 4-V and a good intake will deliver all the power you want--unless you want it to turn 7K, your Air-Gap is a good choice! There is a system I really like in which the "carb" is nothing more than an air valve and the fuel is delivered by injectors mounted at the junction of the intake manifold and the heads. The system looks fairly "period correct" to the casual observers, but those who are gear heads will recognize the fuel rails and injectors. Google Mas-Flo and see what you can find.
I keep hearing GREAT things about the 427 version of the 351-W. I hear equally great things about the 331 version of the 302. Issues related to the 347 version of the 302 involve ring placement, clearancing the bottom of the cylinder bores for the counterweights on the crank, and premature cylinder wear. I'm sure there are companies out there that have addressed those issues, but when I increase my 302 it will be to a 331, hoping to wind it high and quick and last long. The 347 won't wind as quickly, but they really have great torque!
If I were to want more displacement than that, I'd really give serious consideration to a stroker 391 CID version of the 351-C (or, even better, a 427 based Windsor built as a "clevor" with good aftermarket heads....OH, YEAH!!). The Cleveland design cylinder heads would also necessitate new headers, of course. Not many on here will like the Cleveland/Clevor option b/c they are considered NOT to be period correct. IMHO they are one of Ford's best looking engines and they have a unique sound about them. They have great low-RPM torque production but are not usually high-RPM engines.
IMHO, again, I think your least costly option for more power is with better heads, more compression and a healthy cam. LOTS of choices with what you have already. I'd save stroking to be a final "step" with your current intake/exhaust/placement/transmission. Keep in mind you'll undoubtedly need to have the carb retuned for the larger displacement.....they can't just slap it back together and expect it to work well.
Want "cool factor"? Go with stack EFI, they look like webers but have that great EFI driveability
Good luck with your choices.....keep us posted, please!!
Cheers from Dugly
