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

okay, let's start with the transmission. it's early, it's cable operated, some parts are getting tough to come by and they can be a little finnicky. but, that transmission only really works with your shifter. so if you're in love with that set up, you're (basically) beholden to the early transmission. which is fine for the type of driving it sounds like you want to do.

but here in lies the rub: if you use anything besides a 67 and earlier 273 you're going to need the adapter ring for the torqe converter hub to crank register interface. no big deal, somebody here makes and sells that. or, you change your transmission and run an aftermarket shifter-- this is a bit better route in that you get a broader selection of parts availability, part time kickdown/passing gear, and there are more units out there floating around-- literally any 904 from 68 until the 80's can work.

to note, there exists products and procedures for adapting and/or making the later transmission work with the 65 shifter stuff.

as to the motor, i'd probably keep it LA 318 and forgo the 5.2 magnum (or 5.9) dance. as mentioned upthread, a big chonk of your 273 stuff will work with LA 318 where as with the later magnum motor you'll be putting out for a big number of things that are costly: heads, mainly. while it's clearly been done, sometimes just keeping it simple might be the best approach. you can pick up a reman long block for 2 grand or so, snap some bits & pieces on and be well on your way down the road.

exhaust with your build i'd just run stock manifolds, take it as true dual out to the back with some decent mufflers and call it done. if you're feeling flush, lay down the 1K or so for headers that actually fit.

the rear end, you can probably ride it till it pukes and then just throw an upgrade into it. there's reams of info here on the site regarding rear end swaps. it basically comes down to do you want to keep it small bolt pattern or go to big bolt pattern and how much you want to spend or fabricate. this kind of runs some what the same way the transmission does in that you need to pick a direction early on and then build toward that, otherwise you wind up spending the money twice.