If i remember right, the inlet manifold bolts on the carb models are really hard to get at, and very tight, and being allen key bolts will round easily.
I think you need to remove the inlet manifold before you can get to all the bolts on the exhaust manifold, if my memory serves me right.
I would try and get them off with the head on, if they won't come off then you will probably have to take the head off.
Plenty of WD40 needed

Another tip, try coca cola on rusted in bolts, it works a treat, and no i'm NOT taking the piss
