You said it was not treated real well in your youth. Clean the hell out of the barrel. Micro groove can be a real bear to get clean, lots of soaking with something just for lead, is going to help. My 357 lever marlin was the same way, micro groove is its own special thing, but once clean it should shoot better then you report.
I just did springs on one of my 60's at home....currently together waiting for two E-clips that went flying off into never never land....figures. It is not hard, but not real easy, take LOTS of photos you will refer to them putting it back together two weeks later like the idiot I am taking it apart before the parts are in hand. I think the spring kit I bought covered all springs but the mag tube follower. I think it was like $30, no biggie. I can't report on how it helped as...well those _________ eclips.