Hi,
It sounds like the tensioner roller is set up to tight. Loosen this one first, fit the belt and then set it up according to the procedure document found enclosed in post 24 in my project thread here:
https://the75andztclub.co.uk/forum/s...1&postcount=24
The adjustment and set up of this roller is not mentioned either in the you tube video or the Haynes manual, but this is an important part of the procedure. Failing to do so, might result in a failing timing belt as the adjustment bolt of the tensioner roller is only temporarily tightened to 10Nm from the factory.