Two messages---Bert--I've always found ATF applied liberally for a few days usually works to free up manifold nuts. If you can't break the nut loose with a normal 3/8 ratchet (use 6 point socket) don't go for a bigger lever - get heat- heat the nut quickly to cherry red and it'll come loose. If you don't have access to an acetylene torch drive to a shop that does--heat the stubborn nut(s) to crack them loose and then drive home for removal. You will definitely have to remove the manifold--there's no way you'll be able to pry the manifold unless all the nuts are loosened--at that point you might as well change all four "captured" gaskets. Only use gaskets that are a metal sided "sandwich" as original.
Kitty--your dream of a 162 can be accomplished with a '75 164 which has the floor pan of a 240 series from the windshield back and therefore it's possible to mate the '75 164 front to a 242 rear. Years ago an acquaintance built a 165 doing that. Also years ago I ran into a fellow at Watkins Glen who built a 2-door 245 powered by a SB Chevy. -- Dave