The mounts will either be in the correct position or not. 15mm higher is 15mm higher, which ain't really "in the right position" at all.
This will increase the crank shaft angle in relation to the diff pinion angle.
Get these angles outside the acceptable range for your driveline combo & you may find vibration issues crop up.
If you're gunna do it, do it right.
As far as it "working perfectly fine in his EJ", that also depends on what height the rear crossmember sat the gearbox at. Along with knowing if the car had a factory diff &/or if it was lowered with reset springs/blocks etc.
Ensure that the mounts on the replacement HR front end will sit the motor exactly where it sat from factory. It is the only way to tackle fitting a HR front end into an EH.

This may shed some light on the subject. I removed this front end to return to the guy I bought it off as that was part of the deal when I bought the car from him. We later did another deal on some parts and it made it back into my possession. In the mean time I fitted my own hr front end into my ej. I copied this front end exact and it works perfectly. I am running the original 3 speed column crash with the side mounts on the transmission. The car wasnt lowered at all but had the towing rear spring setup if that makes a difference. The rocker cover didnt fowl on the accelerator crossover bar at the rear of the engine nor did the draglink fowl on the sump, it was running a hr draglink. The car was driven without any problems for over 20 years as I know the history of it, it was running a worked hp engine with a x2 carby setup, hope some of this helps