I never heard of a problem with a too thick of radiator.however the thicker the radiator is the less effiecent the rows of tubes cool due to hot air moving past them.I built a 4 row radiator( alot of people call them 4 core but radiators only have 1 core the core is everything except the tanks)for my buddys stock car and it ran cool in 90degree weather.most cars running air cond.have at least 3 rows except newer cars they use alot of aluminum cores and have wider and less rows of tubes is it possible the condensor for the ac could be restricting proper air flow thru your radiator. Im not sure on the cracks but i do know some manufactures radiators back then were very poor quality.did repair several brand new ones