OK, sounds like you may be onto the issue.
The tension is too low to drive the vac pump? really? doesn't sound right to me, check that the belt length is correct. What nominal length are you using?
I wish you luck getting a 4" exhaust through, is performance exhaust the mob in Lilydale?
Spoken to Enkelman & associates, and they are ok with the conversion. Front axle weight will be the main issue according to them. Present the truck with no or minimal accessories mounted to the front they advise.
I will be using them for my project.