Lossen up your heads, this will allow them to move just a tad.Now install the manifold with the carb,(for me it helps to get it seated with the carb bolted to the manifold) move the heads a little until you have a good seal. Wrap a couple turns of electrical tape over the joint over the o-ring (helps it seal, don't worry it won't melt) Clamp it down then retighten tighten your heads. Oh yeah if you can grow a third hand or get a helper it makes life easier'
Sometimes you have to lossen the jugs also but most of the time you'll get enough play by just working with the heads. Good luck
Put late model - 79 onwards - rubber hose manifold seals on over top of the o-rings. Then put those t-bar stainless steel aircraft type clamps over the late rubbers.
This has kept mine sealed good for the past 10,000 miles.
In theory there is no difference between theory and practice. In practice there is.