Until you find something else....
>> issues, only to have it now start dieseling and backfiring through the carb, as well as my headers not sealing properly against the head even with the $50 Remflex gasket sandwiched in between. <<
Not wanting to search your other posts, be sure you remove the short pins on each end of the heads and replace with threaded studs. The holes are already threaded. You should already have two threaded manifold studs. Replace all bolts and pins for a total of eight threaded studs.
Eight studs, the CHEAPEST gasket you can find and DO NOT over tighten.
Expensive, layered gaskets are crap and will leak on your 250.
Look for a gasket like this MerCruiser gasket.
http://www.marineengineparts.com/exh...ld-gasket.html
http://www.oreillyauto.com/site/c/de...2262&ppt=C1980