Check. recheck, and triple check that you found all the bolts! BTDT... Embarrassing and expensive way to find I missed one!
If you can get to them, try double nutting and getting as many studs out as possible. The rusted ones probably won't come out. Put the nut on loosely and tap side to side, try to get the rust penetrant to go down. Some heat on the head might help, but be careful! The more you can move the head around, the more likely the rust will finally give it up.