I have a barebones 68 valiant. I found a 74 dart that could donate its power disc brake setup, power steering and A/C, and electronic ignition. Are all these straightforward swaps from 68-74?

brakes yes, if both SB or /6 yes on the ign just some minor wiring changes & if the eng is different just get an electronic dist for that eng, I think the pitman arm on the '74 PS box will require you to grab the center link/idler arm & wouldn't hurt to grab the tie rod ends/sleeves though I think they are the same but might swap the whole shebang as an assy. On the A/C evap look at the physical spacing between the heater core tubes and where the AC hoses come out of the firewall and compare that to the bare space on the 68 firewall where you will cut for the A/C hoses & see if everything looks kosher to each other & if so pretty sure the evaporator assy is a shoo in but unfortunately I think they will be different.

I forgot to mention, both are /6 cars. And i would be getting the whole car, so i could get everything i need.

The '74 discs will have big bolt pattern wheels & the '68 will be small bolt patttern (if OE). You're good on the dist (I would open it up/rework each (sub)system) & wire in the 5 pin ECU/4 terminal ballast. Pretty sure the firewall layout is diff which'll stop you on the evap but can run a hang on unit under the dash (or find another parts car closer to '68. The aftermarket A/C kits are way too much money

The pre 73 cars used a steering linkage that had the idler and pitman connecting to the centerlink with ball socket studs pointed up. The later cars pointed down. The later setup is my favorite, partly because it makes it much easier to remove the link. Sometimes the link needs to come off to install headers or to access the oil pan.

Sweet! Thanks, everybody, for all the advice. Now i can finally ditch those goofy 13in wheels and puny 9in drums.

One more question, When i change from manual to power steering, can i change the shaft out of the 74 column and put it in my 68?

You can on the early
A's (64-66) & probably 63 (havent done a 63 yet) but not sure on the later ones. I'd be inclined to say no but I do not know for sure. There was a thread on iirc pounding them (later ones) together to shorten it for a PS then repinning it but I did not pay attention