Lol, I did that and the mustang pump didn't fit the holes, so I ordered the 66 dart one and guess what? You're right, identical and it didn't fit either!! Go figure?
hmm I just put my new "mustang" washer pump on my inner fender and it fit perfectly(/6 , 66 dart convertible) lined up good on the wife's v8 convertible too. BUT I tried it on a (partial car) 66 more door and it didn't! 1 hole was off by a good bit, so you got me on that one. I remember going into the junk yards and taking them off mustangs (way back) to put on my older cars 45-49 merc's mainly, coupla dropped fords.) I wish I'd have kept the last 40's merc.. anyway I mounted them near the tail pipe for flamers. always in the same holes as I first made.
and as a gas tank for them I used the Bags that came from some mopars and fords in the yards, talk about UNSAFE but FUN!. (hey I was really young:D and I loved shooting 20 foot flames!) I could get about 4 months out of each one before the diesel ate the o rings. you could spray a LOT of diesel through that baby before it gave out. I always grabbed 3 or more in each yard I went to till I grew up a little. I still had a couple left when I migrated to Mopar and discovered the fit dart pump=mustang pump@1/2 the price.