The P38 engine management system (Gems & Thor) is a complex, closed loop system which automatically adjusts fuelling and ignition depending on the "Live" information fed back from various sensors. When a fully sequential closed loop system is fitted, it forms part of the existing loop and the fuelling etc. is adjusted in the same way as if running on petrol when fuelling adjustments are controlled to each bank of injectors. With LPG mix fed into the Plenum, individual bank fuelling cannot be controlled, plus the variation in fuelling cannot be controlled by the engine ECU based on the "Live" information.
It is well known that P38's can, and do suffer from overheating problems and incorrect fuellin does play an important part in the problem.
Trying to carry out any major engine modification "On the cheap" is only waiting for a can of worms to open.