First, let's clear one thing up, Revell did not engineer this kit--this is Monogram's masterpiece.
Having said that, I have the same issue with the Monogram P-47's. What I do should work for you as well. First, before the engines are installed into the cowlings, tape the cowlings in place on the wings. Then take some "L" shaped evergreen styreen and cut it into 1/4 inch pieces (4 pieces per enging should work). Now insert these brackets through the cowling front so one side is agains the wing "firewall" and the other surface rests against the inside of the cowling. Glue the pieces in place, spaced evenly around the firewall, being careful not to glue the cowling (should not be hard). Once dry , remove the tape and cowlings. You now 4 good mouting tabs/gluing surfaces for each cowling.
Good luck