Have you tried to heat the top of the spindle up? It might be tricky with the proximity to the rear end.
Shop techs will use applications of heat and penetrating oil alternating to heat and quench stuff on 40’s that don’t want to come off.
I’d get a bottle of few bottles of mapp gas and see...
Found myself looking at this company yesterday. Air to water intercoolers. There is a 1HZ build that has one of these styles.
Might take a look into them.
Water to Air Intercooler