I would try a set of ford 071 rs turbo plugs gapped to .6mm and I would check the pipework and the seals on the fuel lines to make sure its not drawing in air anywhere.

It will be one or the other (ie weak spark or not enough fuel)

If its not enough fuel, it will melt pretty quickly.

I would be going straight back to regal though if I paid 4K for an engine conversion and they didnt check the car over properly to make sure everything else was up to the job!

Cant believe he paid that much though, thats FAR beyond what it could be done for you for if you go to the right people!