First because I hate to see the original carbs modified. If you do get a 273 4 barrel manifold you need a 66 or 67, since the 64 and 65 have smaller intake bolts at a different angle.
1. Get a stock 273 intake and use your 4294 carb
2. Get an aftermarket Eddy/Carter AFB w/wide pattern and use the 340 intake
3. Get the adapter and use the 4294 on the 340 intake.
If you're concerned about looking factory stock, then choice #1 is the answer. (If it matters, my choice would be #1)