I think I found the answer to the spare tire location.
" The M1A had been designed to meet the then-prevailing FIA Appendix C regulations, which had been written years earlier to maintain the illusion that the racers were in fact road-going sports cars. The distinctive bulbous windscreen on the M1A is in fact to provide a place to mount the required spare tire. By mid-1965, the new Group 7 unlimited sports racing car rules had been agreed to, so road niceties were abandoned and the distinctive Can-Am shape began to appear in the M1B-wide and muscular rather than small and curvy."


(Ken H)