Trying to get the manifold off our 4age- won't be using it, and its in the way. Unfortunately, 2 of the 5 nuts are rusted solid to the studs (they rounded off when I tried to undo them)... any good methods for getting these off?

Grind 'em off
Take the manifold off. you might find molies on the stud will now extract 'em so's you can put new ones in. Or, if not,weld two shiny new nuts on the stubs of the studs. The heat from the weld will weaken the bond and roberts yer mothers brother. Wind 'em out , get two new studs and upwards and onwards you go !!!!

If you have enough space, a nut splitter will open a rusty nut in seconds, if you are a bit cramped, a nice sharp chisel and a good sharp tap should open it enough to get grip on it to remove