Just do the top end as planned. No need for headers since you have dual exhaust. I have my manifolds Jet Hot coated to keep the heat down in the engine compartment. A D4B has the correct 64-65 intake bolt angle and size. Many times an Offenhauser intake has both 64-65 angles machined and the 66 - up angle as cast. The original "65" 4 barrel intake works well and the original 273 AFB from 65-67 works perfectly as well. No matter what anybody says, early 273's have a deck height of 0 to -.020 and closed chamber heads so you should get quench. They flow very well about .430 lift. A friend built a 2 barrel piston Commando 273 with an Isky E-4 cam and made 265 HP on the Dyno. I also would, and do, call Jim at Racer Brown Cams for a custom grind for what you want to do. He has no problem making a solid cam small enough for a 273. As for the heads, mill them to get them flat, clean up the bowls under the valves, buy stainless steel exhaust valves (If you need any intake valves PM me), Use the Fel-Pro Viton positive intake valve seals and a set of 340 valve springs and a double roller timing set. Get a very good valve job and use print-o-seal intake gaskets. That is pretty much all you need to do. Use brass freeze plugs when you clean up the block.