Cracked it! Ignition Module in the distributer was failing when hot.
Thanks to all those who have contributed ideas and I do believe the ignition module was mentioned as a fix (blackmexyv6) but trying to get them is near impossible for the early distributors. Money is a bit short otherwise a new distributor would have sorted it straight away but hey I have learned a lot with this one - thanks again.
I wouldn't say straight in… but it is the same size externally. You will need to transfer over various parts from your K8, but essentially you should be able to make it work. Not sure about the ECU and loom with manual box though.