I have owned a 1440M for the last 10 years. I use it for duck hunting & fishing around Mobile Bay. It has been a great hull, pretty stable for a 40" width, nice & light and handles rough water better than I ever imagined.
Based on my experience, a 6 hp will not be enough.
I had a 9.9 on it the first year I had the boat. It was fine with just me, decoys and gear. With two guys, it would barely plane off with a full hunting load.
So I bought a Nissan 18 HP 2 stroke for it the next year. That motor has been great, I can put two guys, 6-7 dzn decoys, gear & blind on it, and it will plane off nicely, runs at or just under 20 mph.
With just me & 48 quart cooler of ice & rod/reels/tackle box, I can get 22-23 mph out of it on calm water.
I would not run less than a 15 hp on it. If you can get a used 15hp 2 stroke or an older 20 two stroke if you can find one.
If you find a Nissan 18 hp 2 stroke for sale, jump on it!
I did run a 25hp 2 stroke on it for a while at the end of the first duck season I had it and that 25 was really great on there. But the boat is only rated for a 20, so I bought the 18 to avoid the liability and to keep my insurance coverage.