Final drive calculation.

[The Doctor]

I went from a 4.3 to a 3.3 in my 125 bhp 2T-G KE20, weighing 910 kg.
With a T50 and the 4.3 diff, i could skip 1st gear, and 5th would be 5500rpm @ 120km/h.
Now i've got a usable (but slightly long) first gear and a nice cruising 5th, if i want to overtake, i can go down a cog in the box and it'll pull well enough. Only thing is, my 2T-G hasn't got the power to pull my boxy KE20 through the wind at higher speeds with this diff, thus the actual topspeed being lower then a standard 2T-G powered TE27 was (which should do 200, mine doesn't go past 170 atm), but it's fast enough for me, and the fuel economy is a great bonus as well (it does 1l/11km on a long run, not bad for a 40 year old 'racing engine' on 2 40DCOE Webers). My theoretical topspeed is 260 btw

Your BEAMS has got loads more torque and power, i think you'd be fine with a longer diff, especially with the 6 speed behind it. You can always shift down if needed, for instance, i can go back to 3rd on the motorway if i want to overtake, more then enough power then