I think you are right about affecting casting performance. If the rods are worth anything, it might be better to sell them, and buy new ones. I'm not sure that you need a rod as short as 7'9" for boat fishing, but maybe if you were right in the mangroves and had restricted movement....I don't know, too many bugs.
Juro has more experience with this kind of rod building, so maybe he will chime in.
To minimize the effect on how the rod casts, cut at the butt. A couple of inches off the tip could make the rod more than a weight heavier. Your six and eight weights are probaby the best canidates for this kind of thing, because taking a foot off the butt will still make the rods need a couple of line weights heavier. But the tip will still be soft. These will be VERY fast rods.
Let us know how the experiment works out.
One cast can change your day...maybe your life.