I may be wrong here, but I remember hearing that the stringers are setup different in small block vs. big block 272's, therefore making very difficult or not possible to swap from one to the other.

Anyone else know about this?

Never heard of that before. I tend to doubt there would be any difference. Motor mount location on the stringers may be different but I really don't think the stringers are different.

However, I don't really know for sure.

I did this conversion on my last boat, 85 272ls. From 260's to T340's I converted a pair of blue water inboards. Could only get that heavy puppy up to 59 on the gps. The big blocks made the boat handle badly in rough water. To much weight! But It was all cast Iron back there. The guy with the 540's has the right idea. Lighten those big blocks up so they are at least the same weight as the 260's were. But we do have three of them in my club and none can break 60mph. These boats are made for a good ride, it is an expensive proposition to try to get the to go real fast...

AND HOW...just ask me about it.