The dimensions of the bulkheads are quite narrow on the OVA T4 and requires the bearings to be located more inwards. This requires the TRF 419 gear differentials to have some plastic removed around where the bearing pushes up against. I guess you can call it the shoulder on the gear diff that the bearing pushes up against. Just Dremel that material off from both sides so the bearing would be resting flush against the differential casing. Just test fit as you go.
That is all from what I can remember, and I would have some shims to repositing the diff left to right. You do not want it to be touching the upper decks.