It would make sense to calculate the volume of the kickour and pour accordingly. But I've only done foam under a deck which had several holes to pour into and which expanding foam could release out of.
Still say, cut your new bunks to length, strap them down to the trailer and launch boat, spend a bit of time in the parking lot at the ramp to install. Be my choice. 😀
I might be wrong,but I think Honda made the first 4 stroke outboards many years ago, I had a 7.5 bf75 on my old sailboat in 1976. Probably still running. 🙂