81 Camaro lock up converter

I have an 81 Z28 with a Turbo 350C. Currently, it is causing problems, locking up immediately after going into 3rd gear. Sometimes when cold, it
will lock up and stay that way even if you downshift to first, which is a problem as you can imagine.
The question: Rebuild the 350C with a new lockup converter and solenoid (it's due for an overhaul, regardless of the solenloid/converter problem) or pop a few extra bucks for a TCI or B&M 350 turbo without the lockup? Opinions? One transmission rebuilder told me that the regular 350 turbo is more reliable and tougher than the lockup version (350C). Another told me they are the same, sans the lockup. I'd like more opinions on this. The engine is a mildly modified 350 cid with around 300 hp. I run 1/4 miles at the track on occassion. The rear end is a 4:11, so the lockup would come in handy since I drive on the street and interstate occassionally.
Any input on this subject would be appreciated.
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