So everything was working just fine then, the radiator just sprung a leak.
I measured the core thickness on my Champion 26” 3 row, it is in fact about 2.5”. Tanks are 3”. Currently I’ve got ~9/16” from the end of my pulley to the fan shroud for my electric fans, so, a 2.75” core would be a no go for me. Hopefully you’ve got the room!