2nd gen fmoc Short radiator
 

can the 2nd gen fmoc be mounted under the short radiator from the earlier first gens?

No, the oil line connections will not allow a same fit as a SA/FB oil cooler. You will need to mount it else where. Do a search, I installed one, the thread has detailed pics.
Here's the thread.
https://www.rx7club.com/1st-generation-specific-1979-1985-18/anyone-have-fc-oil-cooler-fb-pics-417817/