First, the disclaimer -- I don't own a driftboat.
But my oldest fishing buddy does (17' Alumiweld), and I spend much time in it. He bought an 80# Riptide about 3 years ago (doesn't hurt to have a line on free, large AGM batteries). It does OK.
He recently bought a Merc 9.9 extra-long shaft.
My thoughts: The electric is cool. We've used it plenty for crabbing, but it's far from ideal. It's best use is on the rivers, and is generally used to backtroll plugs, or the occasional "drift boat drag race to the honey-holes." On many rivers (the Wilson being a prime example), you can only run electric motors, no gas. A big drawback is that once you're "out of gas," you're not pulling out a gas can and filling it back up. When crabbing, it can be quite a struggle to go against a strong outgoing tide. It's very quiet and smooth. Another huge advantage is the perfect reliability -- it starts with a twist of the handle, every single time.
The 9.9 outperforms it in every way. The extra-long shaft eliminates it from being used in shallower rivers (a standard long shaft would have been better, but it was a very good price from a friend). Having the prop lower in the water keeps the boat flatter on the water, which increases top speed (which is much higher than the electric). A spare gas can allows it to run all day(with the large AGM batteries, the electric will run full speed (although 75% is almost as much thrust, while using much less battery) for around 5 hours). The gas is a much lighter setup than the electric, by a good margin (batteries and motor run well over 200#).
By the time a person bought an electric, batteries, breakers (not a bad idea), and a dual charger, they're going to spend as much as they would on a nice used 4-stroke 9.9, which has a great deal more power than the electric.
But both options shine in different situations. If crabbing is the primary motivation for getting a motor, you'll be much happier with gas.
If you do go electric, I wouldn't build boxes. We use the plastic boxes, and they sit on opposite sides of the oarsman's seat (with PVC conduit running under the footrest for the wires), on a rubber mat. Group 27/29 batteries are heavy, and it's nice to be able to pick up the whole box to remove them -- I can't imagine trying to lift them out of a mounted box regularly... sounds like a backbreaker.
Bear in mind, in a driftboat, top speed is very limited. But a good gas motor gets you across the bay, maybe 7MPH if you're lucky.
Enjoy your new boat.
