Vince, the XE270HR is NOT the cam for your 351W. See for yourself at
www.compcams.com. It is for a 302 motor originally equipped with hydraulic rollers, EFI, and uses 1.7:1 rocker arms, and is listed in the 298 - 302 engine section.
Look in the 351W section for retrofit hydraulic roller cams for engines not originally equipped with hydraulic rollers. The cam I'm using is the 284HR, PN 35-442-8. It's a 224/224. There's a slightly milder version just above it at 215/215. One of these will work on your carbureted 351W.