The way I lookat it is, 351W=has potential, 351C=has most potential. Why wouldn't you want the most potential. Upgrade as the $$ comes available.
Dude, I don't know where you got your info from but the Grand Sport never owned the 427 Cobra.
I'm a corvette fan from the early 60s. The two cars were pretty well matched but the GS, more often than not, fell to the cobra.
If there were turns on the race track, 427 cobra's generally lost to the Grand Sport 'Vettes.....that's fact.
http://www.grandsportcorvette.com/gs/
Furthermore, 72 351Cs were still potent but milder than the 70 & 71 4Vs. I'll see if I can locate the info but I'm sure the cams and timing were changed for emmissions. Clevelands have always been an engine that bowtie people have been hesitant to tangle with.