I agree, it sounds like the carby needs a kit put through it and then the car needs a tune.
Our XY with worked 302w, heads, cam, exhaust, inlet, etc runs a 650dp and it seems it is the best carb for the car. We have tried Vac sec 600 holley and it was not the same car.
I would think a 600 Vac sec holley would be fine for a stock 351, but if you have heads, cam and other mods then it might not work that well.
I paid $350 for a reco 600 Vac sec for my 289 Mustang. I think a kit for the 650 would be around a $100.
Used to be Mustang1966coupe.
66 Silver Frost Mustang 289 Windsor
71 XY GS Fairmont in Bronze Wine 302 Windsor
92 EB XR8 302 Windsor
Sure do love those Windsors