are you sure everything was at TDC when you tensioned the belt? maybe it skipped a tooth because you're using the wrong procedure, are you using a manual? IIRC theres a few steps and rotations you have to do before tensioning the belt

FYI- I'd hate to be the first to say this but the P72 will yield a higher compression and has similar flow characteristics to a pr3 head; so I wouldn't just junk that b18c1 head.