What engine for my Wife's 70 dart

I'd throw the standard 318 in with a few basic upgrades:

1. Coil / Leads / NGK plugs
2. Intake & 600cfm 4bbl
3. Headers

That should get the car running easily. You can even just use all the stock gear (the above is in order of preference) but I would definitely put a true 12v coil in it so you can ditch the ballast resistor and get better starting. New battery/starter leads will help here, too.

I have found the Holley 'Street Avenger' 570 to be a great mild carb for most stockish smallblocks, gets reasonable economy out of the box, too. Otherwise a standard 600cfm Holley with 64 man jets, 8.5 power valve and plain (also called grey or standard) vacuum spring will do.

If you find it bogs with full throttle then try a brown or black spring to delay the opening of the secondaries.
If you feel the secondaries kick in they are not opening soon enough, try a pink or purple spring - you will be able to get a 'vacuum spring kit' for not much money which includes the most commonly used springs.

Hope that helps.