Transmission Cooler?

I was told by Dynamic converters that I should run an auxiliary transmission cooler in addition to the one in my stock radiator when I bought my 9.5" Dynamic converter. I've heard good things about the stacked plate coolers that B&M makes. While looking through Summit's website, they have a TON of different sizes and thicknesses available. What size would you guys recommend for a mild 340 w/ 727, 9.5" converter, 3.91s??? Car will be mostly street driven.

http://www.summitracing.com/search?...scending&keyword=transmission cooler&N=400004

Can you go TOO big on a trans cooler?