will they work?

Those rims, if they're for a '10-'14 Mustang like they appear to be, will probably have too much offset. A small spacer will probably work in the front, but the back will be way too much for a stock rear. If you're going to do an 8.8 they might work, but we'd have to know the length of the 8.8 and the actual offset of those rims.

The 255/40/19's are also 27" tall. It will work in the back, but in the front its not going to clear. About as much as you can get away with on the front of a stock A-body is 26". Maybe a little more on a Dart because of the longer wheel openings, but not a 27".

Without the actual offset on those rims it's pretty hard to say.

***Edit***

Ok, those rims are 19x9 with a 42mm offset if they're the original Ford rims. In the front, you'd need to get to about 35mm offset for them to fit. So, ~7mm worth spacer. You'd also probably need to have the hub register opened up to clear the disk hub on your '73, Ford rims usually have a hub bore that's too narrow to fit over a mopar disk.

In the back, for a stock rear end, you'd need about 40mm of spacer. You could get a 1.5" spacer that has the integral lug studs, that would probably be close enough to work. For the 8.8, the width would need to be about 56" flange to flange, or just about 60.4" across the wheel mounting faces.

But you'd still need new tires in the front.