rick laid this down on his screen :
Belt is not adjustable .... and motor position also not adjustable (fixed by
drum mounts) Hence relationship of motor drive pulley to dram pully is
fixed.
The multi-vee belts when fitted will not stay central on the flat drum pulley
..... I can fit central to drum pulley and vee belt is on the first inner vee
of motor pulley, but as soon as it rotates it jumps one vee to the right.

Multi-vee belts are normally driven and drive multi-vee pulleys, I have
never seen one drive a smooth pulley. I have seen the flat back of the
belt, pass around a smooth pulley, such as a tensioner pulley.

Something seems to not be right. Was the original pulley on the drum
smooth?