Presumably this is a race car ?#

ie you had a 4 speed and were happy, or would you prefer 5 ? Does it have to be synchro box ?

Whilst the ET isnt flying ( no disrespect ), if you're lifting the front wheels it suggests either a light chassis, huge tyres and/or very low 60ft's

IMO that would leave me cautions with a T5 without spending a lot of money on it.

I've seen some of the yanks talk about T10's and Muncie's as being cheap strong 4 speeds, maybe 5, but I know nothing of them.

If it is a race car, there are various dog style engagement boxes which will shift much easier and faster than any synchro, or you can have some of those popular american boxes "faceplated" as they call it, which is effectively a dog conversion.

I had this done on my T56, and as far as shifting goes when racing etc...it really does shift much much easier. Road use....it isnt particularly nice though but I'll suffer it.

I've never used the 500 or 600, but the newer 600 is supposed to shift a lot nicer than the older 500, so if going down that route it would be the better option.

Definitely do research though, and phone and speak to a couple of the American specialists, or ask on Yellow Bullet, it's mostly drag racers so they'll know what's available to you.