basic tranny question

[KiltedBanshees93GT]

ok, heres another try, (sorry if there are any reposts). I am planning on replacing my stock t-5wc, and I am trying to figure out what the difference is between the t-5wc, t-5 heavy duty, tremec, tremec 3550? I am leaning toward the 3550 because it is (at least advertised) as being heavy duty. I know that I would need to change the bellhousing for the 3550, and I have read that you have to alter the crossmember, how much modification is required?
All help is appreciated, trannys are my weak point,

thanks,
J