Found my spreadsheet....

So with the data you gave me, the MN12, a 3.538, and assuming a 2700 lb car:
Your peak accel in 1st is about 45ft/s^2 or 1.4G, which you'll probably never get without a slick and drag suspension, which means you'll always blow away 1st even though it'll run out to 50mph on your 255. Just for reference, I'd blow away 2nd in my S13 on street R's with 34ft/s^2 accel.

So 1st goes to 50, 2nd to 72, 3rd to 104, 4th to 149, 5th to 177, and 6th to 266mph.
RPMs drop to 5.0k shifting at 7100 in 1-4, with 5th dropping to 6k.
In 6th at 2k you'll be doing 75mph.

In the race sim part, it's still slower than my old 425whp S13, but not by much. That dyno chart has a ton of area under the curve, even though it's about 40whp light.