The swap wouldn't be easy. It's possible, but the front cover, water paths, and block width are way different. Bolt spacings are close, bore sapcings are exactly the same.
The only issue is rerouting the water supply to suit, and increasing the width from 5.625 to just shy of 7.5".
The 3.2, 3.9 and 4.0 OHC engines are just 250 engines with an OHC conversion. 3.2 and 3.9 OHC cranks interchanged between some earlier Oz 200 and 250 OHV engines.
The OHC block is 1/4" lower to make room for the cam.
It's not impossible, as the 1500 Kent OHV Cortina and Escort engines got a Raymond Mays DOHC head in 1965, and that was just a bolt-up on the stock OHV block. And a Cross flow conversion in 1968.