Fan belt sizing

[bystander]

I agree with Tony, the top pulley is the problem. If you change it for an early coil engine one (the long one - later types are shorter, and will sit further back from the radiator, but would probably be efficient enough anyway)) the problem should be sorted, and the fan belt you already have will fit comfortably. If fitting a reproduction pulley make sure the bushes are bronze and not brass, otherwise rapid wear may ensue.