I am considering buying a 1990 27 Mirage Intimidator with 454 Magnums. How can I be sure they are in fact the 365 h.p. motors as opposed to the 330 h.p. ?. These motors have the rounded valve covers. I thought the Mags had the rectangular valve covers. They do have the the plastic spark arrested covers that say 454 Magnum but that's the only ID on the motors. Does anyone know if these motors were an option on this boat?

Mag's have aluminum intakes and 330's are cast iron. 365's intakes are also a GM high rise design vs the low rise on the 330's. The casting number on the rear half of the intake should be stamped 6269316.

Just be careful that nobody has swapped intakes .I'd pull a valve cover and get casting #'s .Also block casting # on the rear of motor above bellhousing face may help.You may not know if it is really a 4 bolt block without pulling oil pan.A lot of the casting #'s could be 2 or 4 bolt main. http://www.mortec.com/bbc.htm
If it has all the right casting numbers it might be a real mag .Look for the tell tale signs of motor having been apart. In I think 87 merc called 330's in front of alphas , magnums .

Yep . Yank easiest valve cover each motor . Should be 990 or 088 castings head wise .have yet to see someone put rectangular port heads x2 on a peanut port merc .

easiest thing to do is look on the motor rite above the starter and see if the mercruiser tags are still their,,pull the serial number and run them or call bam they can tell you..