All early As with slants had the stupid rod/bell crank setup. It looks like this.
You'll notice just to the right of the bell crank hole on the firewall, there's three black spots. These are the mounts/holes for the factory V8 throttle cable (this is my factory 273 Dart.. has both options). In order to change the stock slant Holley 1920 (I think) or the Carter BBS to anything else except the Offy 2x1 setup (which looks like this)
You'd need the stupid rare '64-'66 factory V8 gas pedal, and throttle cable. Any later gas pedal should fit, but has a different waffle pattern for the purists.. They repop the pedal these days, but not the cable that I'm aware of. At any rate, this really is the cheapest option for '68's ugly duckling, because he'd have to drill holes in the firewall for the hard to find, and pricey throttle cable, or cut the entire heater out of the car, and remove the section of firewall that bumps out into the engine bay in order to fit a V8 in at all. I have worked on a '76 BMW 2002 that has a rod linkage going to the back of a Weber DGEV 32/36 that seems like it might swap onto a slant with modification, but then you're still looking at manifold/adapter/non Chinese Weber purchase, and I'd estimate all of it at around $600 scavenged off of EBay, and Redline.. That wouldn't include modification, or fabrication either.
As for v8 swaps into pre '67 A bodies, if you want to keep it factory looking at all, you'll need a radiator, oil pan, early LA timing cover and water pump (water outlet and timing marks on the other side), motor mounts, exhaust manifolds, modification to the trans crossmember if you want dual exhaust, 90 degree oil filter adapter, (318 M body.. shh.. it's a secret) '64-'65 V8 pushbutton trans if so equipped (good luck), and of course, the goddamned center link with extra drop in it. Why the factory continued to run half of this shit as slant specific after coming out with the LA, I don't know.. I assume that they had another 300,000 slant center links, and gas pedals/throttle rods on the shelf, so they just kept using them instead of switching the slant over to V8 ancillaries. At any rate.. the 64-66 cars are hard enough to find the V8 shit for, let alone the minor firewall modification required for the '63s, or the absolute bush league hackery needed to cram anything into a '60-'62 car.
The slant headers are mostly frowned upon (for street cars) because they don't attach to the intake hanging out in the breeze, and you get shitty warm up characteristics and fuel puddling in the manifold.. I was tempted by a cheap set for a while, but realized that the car would drive like total butt and wouldn't fully warm up on my three mile jaunt to work from my house. "Dutra Duals" are the ideal answer, but he wants about $400 (IIRC) for his modified casting, which doesn't include modification to your existing manifold, (cutting it in two, and capping the hole in the plenum).
The lesson to be learned here is that if you want an early A body, you'd better be able to enjoy it's Byzantine acceleration, handling, and braking. Or shell out enough cheddar over time fixing those quirks to have bought the '68 small block Dart you wanted in the first place.
Whee!