[GTJIM (R.I.P.)]

Tuff

The 1.9 Opel motor has an extremely durable forged steel crankshaft with large journals and a short stroke with five main bearings. However your 1973 motor probably has the heavier, but weaker cast rods. For best strength with stock parts you need to find some of the earlier forged steel con rods.
The stock pistons are cast and not up to high boost pressures - you need to fit aftermarket forged pistons. The other problem is cooling - the later head such as you have is well known for cracking when subject to high heat flows.
HTH

 

[GTJIM (R.I.P.)]

So ......

Parts book doesn't give a clue, but I thought only the '75 FI engines used cast rods . . . wanna jump in here, Bob?!

The change to cast rods was not contemporaneous with the change to lo-compression then? Every motor that I have dismantled ( seven of them ) dated from "74" through "79" has had cast rods. The only earlier motor I have is the original from my 1972 GT - and I have not dismantled that one yet but it has 9:1 flat-top pistons as I can see the valve notches through the spark plug holes. The PO told me she had it upgraded from lo-comp soon after buying the car due to lack of power - the dealer fitted a Weber 32/36 at the same time!
Strangely enough the later motors I have dismantled are all flat top pistons too - all from 1975-79 Holden Toranas - so not quite the same as your 1975 and earlier motors over there.
So ..... Turbocharging the stock lo-comp engines from 1971-74 would be an option as with 7.6:1 compression and forged rods they would be good candidates for this upgrade??? Except for the head heat flow concerns??

As for stronger rockers - have a search for "roller rockers" on this site.