alright, i swapped a gt40 upper and lower onto my 91 5.0, started my car and its knocking super hardd. drivers side sounds like piston slap. what did i do!??

 

did you maybe drop anything down the ports or into the runners? like a bolt? a socket?

 

take out sparkplugs and make sure none of them are damaged even if you're "sure" nothing fell in there. Spark knock is a coffee can 1/2 full of qaurters, knocking is a thrown bearing

 

oh ****. better take a look. seen people drop things in the engine and never know they did it.

 

were you missing any fastners when you did the job....washers, small screws bolts, etc. If you missing something and with the noise I'd get one of those small flexible magnets and take a look. Or alternatives remove the exhaust off the head and see if anything it in there while rating the engine off the crank by hand. Just a thought.