This one is rated at 20,500 GVWR and is like the thin one.
http://derale.com/products/coolers/transmission/series-8000-plate-fin2013-10-28-09-53-27471243954/black-tornado-electric-fan-mounting-bracket-kit-dual-fan-detailThis one is more like the thicker one, and also is rated at a similar weight range. 22,000 GVWR.
http://derale.com/products/coolers/transmission/series-9000-plate-fin/180f-fan-switch-thermostat-detailSo it seems like the best one will be 1-1/2" thick and 12" square - if that will fit.
http://derale.com/products/coolers/transmission/series-9000-plate-fin/190f-fan-switch-thermostat-detail It has a 24,000 GVWR capacity.
So I would go with the thicker one. Both will hold about the same amount of transmission fluid, and do equal cooling capacity.
Fred.