McCain may be pro-life, but (1) he "owes" Bush for slights real or imagined; (2) he likes being a star and media fawning; (3) given his pride in McCain-Feingold, he has little incentive to want strict constructionists on the court (he probably read Scalia's dissent on the decision upholding it).
McCain would go against the Constitutional/Nuclear option, but I don't think he would vote against one of the Presidents nominees - especially if he had voted for them in the past.