Do I have messed-up injectors??

Almost two months ago I changed my lower intake manifold gaskets due to a leak. The job went fairly smooth and I used the good Fel Pro Permadry kit. I had a few semi-related issues with some parts after re-assembly, but Old Master helped me through those questions. Thing is, the van never was quite the same after this gasket job. I get a rattling suction engine noise only under slight load (clears out under increased throttle), that was never there prior to the gasket change, and my gas milage seems to have gone down a bit. I re-checked all gaskets fearing an air leak, but all is ok. The Fel Pro kit included new upper manifold and valve cover gaskets as well, so I used them. I installed all new parts except for one large and one small "o" ring, that I couldn't figure out were supposed to go. When I had the upper manifold (plenum) off I inspected it for any obvious problems (it is the old style spider), cleaned with some Gumout then set aside. I have recently read that this was a no-no and that I should have tried to keep all the injectors wet and capped off, or damage will occur. Over the past few days I have done a Seafoam treatment to the intake throuh the pcv, and have noticed a smoother acceleration and less of the noise. I don't have an injector pressure cleaner or I'd try it. I dumped some Techron in an almost empty tank, but really didn't notice any improvement. Can anyone give me some injector knowledge on this one - Could I have messed them up during my repair, and would a slightly stuck injector cause my sounds I described? I know a foul injector will lower mileage, which I do have.