Also bear in mind that if you lie an SLA battery on it's side the 'bottom' end will usually be heavier than the 'top' end. The weight is not evenly distributed!
Colin
A very good point often forgotten. The top of most gel batteries,down to a depth of about 10mm, is mostly fresh air! This is why a 6v 10amp laid down on its side in the Type 22 would need trimming weights to compensate for the weight of the battery being off to one side. The 6v 4amps suggested will only lay down fore and aft ( don't stand them up!! ), but you could change their orientation fore and aft, to effect slight trim changes if needed.
On a related point,the model will still need lead, right down in the bilges, and to be honest,if it was me,I would use one or two 5000mAh NiMhs,rather than gel batteries. This will allow you to get all the weight right down in the bottom of the model. You want stability to be front and centre your main priority with a long skinny, high freeboard model like this. Better to get the best performance possible, than be watching the weather before a sailing session, in case it is too breezy!