If they are bad and rotten id suggest cutting large chuncks away for easier removal. get a spot weld drill bit too and have alot of patience. They will be hidden in places you wouldnt think.
Also, before you remove them, take some measurements of where the core support is as its a pretty flimsy piece of metal without the fenderwells holding it. Id go off the cowl area to the core support myself, and mark where you measured so you know where to remeasure it later. Im getting pretty good at taking the a-body ones out now, i can have a set of within the hour. Also, im going to assume your engine is out? If not, it will be impossible to get to all the spot welds. good luck!!!!