I also swapped a 5.0 for a 393W.
I pulled the T5 and set it aside, figuring that at least it has some resale value if I don't blow it up.
Bought my Tremec from Mike Forte. You might shop for a few bucks less, but his service and attitude are priceless. He ships UPS, not truck freight.
You'll also need the bellhousing and 3 qts GM Synchromesh
oil.
If you've ever been unhappy with that big RPM drop going to 5th gear with the T5, you can buy the 3550 II (or TKO II) with the .80 5th gear and a higher torque rating. Then 5th becomes just another gear, without the big drop.
HTH, Ed