A833 4-Speed on a 4-500hp 440: 18 or 23 Spline Necessary?

my bud is a saltflat racer , not a member here . these were the one tranny's that would hold up to foot on the floor n grab'n gears behind a 800-1000hp hemi's . 392 with + 30 lbs boost from a 14-71 on that hemi 4spd mga with 18'' added to the length went 265+mph on 5 mile course on the salt . his twin turbo'd twin 500 caddy liner went better then 300 mph same course . could not back it up , do to a body fire from turbo heat . nearly took him before he got it wow'd up , benders only , parasute burnt off do to the flames trailing the car . new design on the new one .