The .990" OD matches not only the stovebolt but also the Buick L8 IIRC.
However: the GMC doesn't have hydraulic tappets, but the 1958-63 Chevy uses the hydraulic "waist" to transfer oil pressure across the tappets front to rear. If this is missing or in the wrong place, the rockers get no oil.
Grooving the tappet bore works, but it's delicate machining (must be absolutely parallel to the bore, concentric and clocked to within 1°?). I'm not sure the Chevy has the same amount of material in the gallery.
They're also very heavy, which means really big springs (300+ lbs.), I'm not sure the cam drive will like this long-term.
Grinding the original cam to a roller may not work, the stock lobe is very short and "pointy", leaving little room for a new and much more "square" lobe.
