No clue on the tf's, but the thm's names represented their torque capacities (or so it was claimed) IE: th350, 350 ftlbs, th400, 400 ftlbs (even though they will handle much more in factory form). Think of the old Buicks, Caddys, and Olds' with engines that put out close to 500+ ftlbs and weighed 4000+lbs, lol. One thing about the 400, it takes more power to turn, and it has a bit higher geared first. Your eagle will be more sluggish and fuel defficient than ever, lol, but as long as you set it up right and don't do neutral drops or any other torturous things, I'll doubt you'll ever break it.