Magnum Heads....

I have a 69 dart w/ a 318. I have a procharger going on it with a set of magnum heads that came off of an 01 Durango.

Heres my question: I sent them to the machine shop to have cleaned up and but bigger valve springs put in them. Well after they cut the spring pockets they realized that they couldnt find any locks to fit the valves... so i ended up putting a set of stainless 2.02/1.60 valves in them.

Some one just told me yesterday that those heads get their strength from the valve seats? and if you open them up for larger valves, they will crack.. This same guy said he did the the same thing to a pair of heads and ended up crackin 5 chambers from the intake valve to the spark plug hole.

Dose anyone know anything about this??

Thanks