switching to roller

OEM roller 5.0's are drilled all the way through. Not sure about 351's. Engine needs to be out and dis-assembled really. If not, then you run the risk of drilling into the cam bearings. Not to mention the metal shavings you may miss.

If you drill all the way through, measure for bolt length. Too long and you will bind the cam up and wipe out some bearings.

the lifter bores on that old 351 are to short for the factory roller lifters , the later blocks have longer lifter bores, to run those lifters in a early block you have to run a reduced base circle camshaft , which will kill performance , you should use the roller lifters with the tie bars !