how much would it cost to have a 351 windsor installed in a 1980 capri

I'd do it for $600.00 , but i'd also recomend a 302 boss instead, i'd be a tight squeeze , jus watch the exhaust routing as could present a fire hazzard;)

This is not a casual fit. Assume you can find someone to do it, a pro would charge upward of 2000.00. A good shade tree guy might do it for several hundred. You and some friends could do it for 2 or 3 hundred and some skin off your knuckles.
My brothers and l have done this sort of thing. A torch and welder will probable be needed.

That car is too small for that big block engine. Even a 302 is pushing it so l would go with a 289 because of the size of the car. And that would cost anywhere from a few hundred dollars to a few thousand dollars depending on where you go and the experience of the person you are allowing to do it.