you can change the manifold gasket from the top, but it is usually a waste of time to start from there.
the bolts will be rusted so badly they will snap off, then you remove the dog house and try to drill them that way.
best way is to just pull the head, use some heat on the bolts to try and lossen them.
that way when they break off you will already have the head off and can drill themout yourself or just run it to the machine shop and let them do it.
you will probably find out the gasket is good, [or the manifold bolts are already broken off in the head], and the manifold is cracked underneath.i would be looking for a manifold before i tore it down.
sounds like the carb baseplate is worn out and needs to be rebushed around the throttle shaft. the "sticking" is the throttle plate binding in the bore of the carb i bet.
make sure all the linkage is free and oiled good before removeing the carb.