Hi there Dan
I have built one of these models plank on frame many years ago.
Originally the model suffered a similar problem - i.e. as though it was top heavy.
What I did was added lead shot mixed with resin in the stern of the hull round between the two prop shafts and at the bow. This was a big mistake due to the fact that I couldn't remove ballast later on. This was because originally I was running it on 6 volt 1.5 amp lead acid batteries - the thin ones - which used to sit really low in the centre of the hull on their sides. When I came to upgrade the battery to a 6 volt 10 amp lead acid battery, it sat too low in the water (above the black water line) but, I don't know how you are mounting your SLA battery in the hull, BUT, do you realise the battery is heavier at the bottom than at the top - therefore, if the battery is standing up in the hull, you still have a fair bit of weight at the top of the battery which is acting as a fulcrum lever - and this will add to the dis-stability of the hull.
You could try, lying the battery flat and using a small amount of weight to counteract the heaviness of the bottom of the battery. The other alternative is, which I have used now, is this type of battery (see picture) I use two of them in HMS Exeter. Have you noticed what I do, I built a tray for the batteries to sit in and at the bottom of the tray there are two brass strips - these are positive and negative pick ups - which go to the speed controller. On the batteries there are 2 corresponding copper strips which make contact with the strips in the tray. The only thing you have to be careful of is to MAKE SURE THE BATTERIES ARE IN THE SAME polarity - in other words don't short em oot.
The last thing as far as aerials thing is - I use 2 small beads on the ends of the aerials as protectors - this is because I was told a horror story by a modeller at one of the shows about a guy who bent over a model submarine to inspect it and he was stabbed in the eyeball by one the antennas. Still to this day I cringe about it.
Here are couple of pics -
1. I think this is of your model and battery set up
2. One of the model that I made many moons ago
3. The battery set up in HMS Exeter - just to give you an idea.
Last but not least - is the funnel on your model solid resin? - if so, it may pay you to try and drill it out to lighten the weight.
Aye
John