it will lower compression and create a slot that carbon can build up in. One of the members on this forum tore his 350 down to find the large bore gaskets. It was a mess, you might be able to ask him for pics, His name is Kelly(
http://forums.maxperformanceinc.com/...r.php?u=136951) RKM1003. It seems it gives the engine a weak spot that can be chipped at by heat and pressure.
Always good to do things right. It's a big difference. I have a new set of 350 head gaskets from an NOS gasket set. I could try to find a 400 gasket to lay over it so you can see the difference. A 350 gasket is about 3.9" and the universal is 4.25. That's over a 1/4 inch! The only place they are the same is at the valve reliefs 3 and 9 o'clock. Now that I think about it, I have 2 sets of 350 gaskets.