I take it you mean a gel battery ?
In theory, the varley red top is designed specifically for automotive use as this has a unique set of application problems - namely wide temperature range, high surge currents out, 'fast charge' voltage level only with curent limit mich higher than normally used to charge batteries.
In practice, I have been using normal 'leisure battery' type gel ones in my cars for the last few years with no issues. I have a ready supply of these secondhand near end of life (once capacity is too low to last a full day in our electric vehicles). Since they don't get discharged very much they still last quite a while when they are otherwise scrap. (my pug has had the same one for about 3 years now). I'm using 60AH ones mostly, type 12TLG60
http://secukbattery.co.ukI've been running a small 20AH gel type in my 185 so it stays in the engine bay. The only issue I find is the voltage isn't that stable when idling (varies with alternator output) so relies on the injector deadtime vs voltage setting being accurate. All part of making it difficult to get a stable idle with cams / big injectors / link G3.
Bear in mind gel batteries tend to be very fragile electrically. If you let it go flat, it's scrap. Avoid buying secondhand for that reason.
As for location, electrically wise it wants to be as short a cable run as possible. Within the engine bay is ideal. Weight distribution wise, CofG is towards the front as standard so would be aided the most by moving weight as far back as possible. Moment of inertia wise (resistance to turning and stopping turning i.e. turn in understeer, exit oversteer) the further from the CofG the greater the moment of inertia. Like everything, it's a trade off.
The other advice for safety, is to put a fuse as close as possible to the battery, as ANY cabling between battery and fuse shorting to ground will catch fire. For round terminal batteries, you can get terminal connectors with fuses built in (look at gtfours relocating battery section). Not sure about screw in type ones.
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