They would have to be an almost giveaway for me to even consider them. You had big blacks in your boat. Every bracket, mount, wiring, flexplate, etc is going to be for big blocks. Small blocks you'd be starting over, then wanting to change someday.
A apir of completely stock 454 marine engines would put you more ahead than a pair of modified small blocks. I'm assuming you plan on keeping the boat for life because of its importance, so tweaking the big blocks over the years will be a much better overall plan than a conversion now, then a complete re-conversion someday down the road because of the limits of the small block platform.
Now, this is my opinion, and I have been wrong before........