As above a HR prefixed 4.2L engine is from an LH or LX, same engine as what was in a Holden at the same time just the prefix plus a few minor differences like rocker covers.

2.78:1 Salisbury was gone by HZ, replaced by 2.60:1. The standard diff for a 4.2L manual HZ at HZ release was a 3.55:1, replaced mid HZ by 3.36:1. If the engine is modified to HQ-HJ 4.2L power levels you will be fine with a 3.08:1 as this is what was standard for 253/4.2L manual from HT-HJ, only replaced by 3.55:1 in HX as the pollution 4.2 was so badly affected by the anti-pollution build of the engine.
A 2.78:1 behind a 253 and M20 will be a clutch fryer, unless the 4spd has been modified to a wide ratio (M22) aussie 4spd.