For shaft length, you need to find out if the kicker drags when it's tilted up and you are trying to get on plane and when you are on plane. If not, then it's fine (use what you have).
For too much weight, are the two smaller black holes, which appear to be about 4 inches below the blue paint, drains for the spashwell? You need to find out if water comes in instead of draining out when the boat sits in the water.
If you come off plane and water comes into the spashwell or if a wave comes over the transom when you are at rest jigging, it needs to drain out before you get in trouble on the next wave/s. Or what happens when a fat person stands in the stern?
I have a similar set up in fibreglass and am okay based on my answers to these questions.