I think you can pretty clearly trace western political conservatism back as far as, at least, the French Revolution and those who wanted to maintain (and later, return to) the Ancien Regime or, at least, some form of absolute monarchy. But I also think that Buckley could be credited with the creation of a peculiar strain of "modern American conservatism" (or perhaps you could say "liberal conservatism" or "constitutional conservatism" to your point) that still preached the value of traditional institutions but did so in the context of the liberal political framework of the constitution. In other words, he essentially cast the Constitution (which was certainly not a conservative document at the time of its drafting) as the framework for the protection of classic conservative values.