Unless the crank has been 'Tufftrided' or 'Nitrided' there is no need to re-treat after polishing - either of those surface treatments should be done AFTER polishing as they are very thin layers of hardening.
Shot peening - on the other hand - is not done on the bearing surface but concentrates upon the fillet radii to compact and harden that high stress area. Needs to be done by a VERY competent shop though ... just bombarding a crank with steel shot is NOT 'shot peening' - it is much more technical than that!
The 2.2L stroke of 77.5mm is the "Sweet Spot" for Opel crankshafts giving more capacity without the penalty of rods hitting the lower cylinder bores or making a long-stroke 'Tractor Motor' out of the engine .... Go for It!!
Mind you 79.85mm might be better .... "Nah! It's STOCK ... 69.85 is a misprint!
