Swansea - Manchester is about 220 miles
Cardiff - Manchester is about 175 miles
Cardiff - Swansea is about 45 miles
Manchester - Crewe is about 35 miles
and the range of the 67s is about 800.
On that basis a diagram such as:
Cardiff - Manchester (175)
Manchester - Swansea - Manchester (440)
Manchester - Crewe (35)
Is 650 miles.
Switch the last to Manchester - Cardiff and that's 790 miles, just within the range of the 67.
The issue is that type of diagram doesn't give opportunities for maintenance swaps as it starts and ends at the same location, canton in this case, along with having the Holyheads in the mix.