Sounds like it may have blew an intake or carb gasket when it backfired. My brother has a ford ranger that did the same thing a couple months ago. His is a 2001 with a plastic Intake, when it backfired it busted the intake!
this is an 87, it has a 351 windsor in it with aluminum intake (edelbrock performer i think)
the intake is definitely still intact and i replace the carb base gasket when i pulled the carb
i was hoping it was just a small vacuum leak, but im not sure what it is, and i just had it with this truck
a brand new engine shouldnt be giving me this much trouble