Replacing chassis battery with group 27 batteries

Let's See
New alternator, new isolator
You drive, then it won't start,
You start generator and then it starts
Shut off engine and it won't restart, with out generator

Providing it started find, before the new parts where installed
I would guess that something is wired wrong
If the reason for the new parts was the starting problem
Then I would have the battery tested
A new 8d will be stronger than any pair of group 27s

Although my 5.9 Cummins isb uses 2 grp27 batteries for the engine/chassis