Stop-leak, in any form, is the debbil.

While I am sure someone will chime in with how their grandfather used it and in some vehicle and it is still running strong 50 years later...

But, the stuff is designed to lodge in small places, and it does not discriminate between the crack and other places in the cooling system. Sadly, in the Isuzu 4-cyl, this means the coolant flow through the head is further affected. In the end, if/when you go to do the head/gasket in the future, you will have to make sure you get all the crud out of there. This includes the radiator and the heater core.

It may prove out to be more of a headache in the future than you had planned.

Good Luck,
Michael