These are only my unprofessional opinions...I don't think vapor lock is a concern. Mount the fuel block wherever it looks best to you.
If it were me, which it isn't, and I was bound and determined to run three carburetors, I would spend some time to find three matched units in a very small venturi size and run them off a common linkage.
To be honest, your engine isn't modified enough to support three carburetors, again, my opinion. A overbore without some head work and an accompanying cam doesn't support the added fuel.
Two carburetors is pushing it and three big fat Rochesters will drown you out. If you want to run three functioning carburetors, they need to be as small as you can find. This won't be cheap! Maybe the best plan would be as HRL mentioned, block the two outboard carbs off and run only the center.
I sure don't want to rain on your parade, but having a setup that might be designed for failure is not going to make you happy.
Jon (carbking) has the expertise here...give him a call, or maybe he will see this and offer some advice. Keep us posted and good luck.
Stuart
