I have to confess that the only version of this song I have ever heard is Donovan's. I have always followed his arrangement, which is in D.
Intro D//A7//D//G//C//C//C//C// A hundred and (A7)eighty were (D)challenged by (G)Travis to (D)die; with a line that he (A7)drew with his (D)sword as the (G)battle grew (A)nigh; the (G)man that steps over the (D)line is for glory, and (A7)them that is left better (D)fly; and over the (A7)line stepped a (D)hundred and (G)seventy (A)nine; (G)hey! up Santa Anna, we're killin' your soldiers be(F#m)low, so the (G)rest of Texas will (D)know, (A7) and remember the (D)Alamo, A7//D//G//D//D//D//D//
You really need to listen to Donovan's arrangement, which uses a walking bass line to provide the interest and continuity.