Sorry Silverstreak, you do not seem to know to much about the RB.2-11.
There is no way that you can fit a B767 or B747 version of the engine onto a B757, they are a different variant.
The thrust rating for the B767/B747 engine is way higher than that of the B757 and l would guess that only a small percentage of parts are common.
What l can remember some of the RR engines on the BA B767/B747 fleet are a hybrid engine with some of the core from the trent engine.
I am sure someone from BA can confirm this.