boosted,
Sorry bro, I had a nice reply typed up, but apparently didn't send it. :-/
My (Huge) core is roughly ~ 23.5" x 12" x 3". I would suggest something shorter like 10.5" tall at most.
Use these designs from Spearco for rough dimensions.
http://www.turboneticsinc.com/node/81
You will have to decide how you want your placement of the end tanks (equilateral / centered, or offset). Keep your connections at 2.5" OD max.
I think something along the lines of the Spearco 5-312
- Core Dimension
3.50'' thick x 7.9'' tall x 24.0'' wide
- Hose Connection – 2.5'' OD
The reason that I am suggesting all of this is because I went with one of the known 'kits' and had to rework every single piece. And my FMIC rubs the inside of my bumper on to AND bottom.. and I have it recessed about as far as I can get it without performing a lot of 're-working'.