I run the 404 Extremes but now that I look at what you are running you may want something more than V rated tires. The NT 555's come in 20" and are Z rated though. Here are the specs that I found.

404 Extreme

*255/50 R20 109V 11.5 29.92 10.24 7.0-(8.0)-9.0 2271@50 694
*275/55 R20 116V 11.5 31.89 11.10 7.5-(8.5)-9.5 2756@50 651
*305/50 R20 120V 11.5 31.89 12.28 8.5-(9.5)-11.0 3086@50 651
*305/40 R20 112V 11.8 29.61 12.17 10.0-(11.0)-12.0 2469@50 702

555

*275/35 ZR20 102W 10.6 27.60 10.91 9.0-(9.5)-11.0 1874@50 753
*275/30 ZR20 97W 10.6 26.61 10.71 (9.5)-11.0 1609@50 781

My bad I thought that the 555R which is a drag radial came in 20's also.