For anyone else that finds this thread with a similar problem - issue was lower o-rings on the RHS injectors. Both had split at some point and drying them out/moving around while removing the mani must have done them in for good

Replaced with new o-rings and isolators and all good now (Kit ID# IRK718 for the yellow side-feeds), thanks for the pointer Rossco. On to the next..

Cheers
Jadyn