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I thinly you should use MT ET Streets. The radials still spin.
http://www.summitracing.com/parts/mtt-3755/overview/

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Not true. Plenty of cars on M/T ET Radials going crazy fast, hanging the hoops. I run a set of 255/60 ET Radials and dead hook with 1.63X short times. Key is correct pressure. I have great results at about 19 psi.

I think the OP is a little ways off from holding a 12 second slip. But getting there is what this is all about, right?!...
A converter would be money well spent. Even with the short exhaust, the mufflers will cost .2, So I'd find a way to uncork at the track. Depending on what you currently run, .020 shim head gaskets might get you a little compression, which equals horsepower. Good luck, and keep us posted on your progress.

Oh, by the way: My guess is you're currently in the neighborhood of 13.40's with the combo you describe, but good tuning might get you closer than I think.