Wide Fronts on a 68 Dart

Perfect. Luckily I will be running 3 piece wheels and can always go wider later. Is 18x9 widest without tubular uppers?

Should work, yes. My effective offset is 27mm, but I have more room to the fenders and I haven't rolled anything in the front. My own personal preference is to set up with about 5mm of "extra" backspace, because you can always add a 5mm or smaller spacer if you have too much backspace, but you have to cut or roll or buy new wheels if you don't have enough.

So, yeah, I would look for an 18x10 with about a 30mm offset and plan on a small spacer with the stock 73+ disk set up. Which might also allow you to keep your wheels if you upgrade the brakes later. A 25mm offset should work, but you don't have any wiggle room if it's tight on your car. If you're planning on aftermarket disks, you probably want to add their offset in before the spacer. Which is how I ended up at a 35mm offset for my set up, because the dr diff kit I run adds ~5mm per side.

And don't forget you'll need tubular UCA's to clear rims that wide.