273 swap to 318/ build, cost, potential, daily driver

First, you need to decide if you're going to keep that one year only transmission. If yes, it will need to be rebuilt.....and maybe beefed up a bit if you decide to add horsepower over the 180 the stock engine had that year. If you're ok using a later transmission, you can pretty much forget using the stock shifter assembly. That is either going to mean an aftermarket floor shifter, or, a '66 only A-body setup. That would mean a complete column shift column and all the linkage, or, a complete floor shifter, floor brackets that need to be welded in, console, and also all the floor shift linkage. If you're going to keep the stock trans and shifter, I would just find another 273 or early 318 to build. The advantage of the 273 is that all your existing engine parts will fit it, and it's compatible with your trans without any mods. The 318 will need the crank bushing for the stock converter, but otherwise will accept all your 273 parts also. You could also use the later and easier to get intakes on a 318 or '66-9 273. You get into 340's, 360's, and magnum stuff and you're also looking at oil pan issues, and modifying motor mount brackets to fit, along with exhaust and other nits. Look around and see what engines and transmissions are available, the cost of trans and engine rebuilding and see what fits you budget and final desired results.