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Find out where all the local hotrodders hang out and go find out where the best local trans shop is.Also some good vids on youtube on the 4l60e.There are some upgrades that can be done while the trans is apart.
Before I ended up having my 4l60e rebuilt locally I also looked at several of the trans shops that advertise nationally including the one you mentioned. I urge you to read the fine print carefully or speak to them on the phone. What I discovered is that if you purchase a rebuilt trans from them you get NO warranty what-so-ever unless you also buy a new TC from them at the same time.
ODXR had a good suggestion. go to where the local hot rodders hang out or visit some corvette specialty shops and simply ask where people get their trans rebuilt locally.