Jerrys Gaskets will have the new O-rings for the injectors. Only the 90 ZR-1s have upper (injector) body gasket O-rings on the PRIMARY injectors only. The rest are sealed at the tip. (Jerry knows exactly what you need - just tell him the model year. And, he can set you up with new injectors too, if need be!)
Far as the IH goes, all that may be needed is to re-torque the bolts holding it to the block. A couple of mine were barely finger tight, first time I pulled the plenum! Also, some BLUE LOCKTITE thread locker on those bolts will keep them from loosening up again.
And, just a suggestion too.... unless you drive you car in sub-zero weather, you can BLOCK the coolant ports leading to the plenum by drilling and taping the holes and inserting a (screw-in) plug. Then the next time you pull the plenum, you won't need to worry about draining the coolant! (Mine have been blocked for at least 10 years now, and not a whit of trouble! 'Course I don't drive it in sub-zero temps either!)
And, here is a link to
Del City which carries just about all the connectors (with maybe the big one attached to the Digital Ignition System (DIS) module).