It looks like you are going to be fighting rust also. Try taking a punch and setting it square in the cross of the philips head and smacking it a couple of times really good with a heavy hammer. The shock of the blows will help loosen the crud around the threads of the screw. And I still recommend the prick punch method, if they get stubborn use a chisel at an angle.
The first time I tried to remove those screws I the impact driver which just stripped the bit index. I almost got drastic and welded nuts to the heads to remove them until I tried the punch method. But I was trying to save the stock screws. If you are planning on just using bolts, try the weld on nut method.
Take a nut and weld it to the head of the screw from the inside of the nut, then just grab a socket that fit's the nut and a breaker bar and them screws will come out as you have added enough heat from the welding to help loosen them and can now get enough torque from the nut to trun them. And the welding method causes less damage than grinding and just using heat from a cutting torch.