305= misfire #5 cyl, 306 misfire #6 cyl. These are related! 1518 intake manifold runner is not! Either of these will turn on check eng. light!
Find source of misfire(s) and reconnect IMR linkage, most likely a broken linkage rod retainer/bushing.
You didn't cross up any plug wires did you, cause these 2 cylinders are next to each other! If in fact you did the work??
Recheck plug wires for proper routing to the proper cylinder. May be crossed! These are the two rear cylinders on the drivers side, front to back is 4-5-6.