You mentioned that the early 904/a999 had hydraulic type Lock up? Do the newer models have an electrical, solenoid type switch, like the A500/A518? What should I look for at the junkyard? year, external plug connector?
You are making the right decision.
The Locup trans already comes with a slightly higher stall than the regular 904. I'll guess ~300rpm.
The later A999s, I have read, had the loc-up electrically controlled by grounding the internal solenoid. You can install a dash switch to do that. So that opens up your choices to all A999s. The TC for the A999 is different from an A904, and they do not interchange. So you kindof need to get them together, else you will have to buy one. Hi-stall lock-ups are sortof a specialty, so not cheap. Better to get the TC with the trans.
Another option is the A998, non loc-up, then a regular A904 TC fits. But you have to be really careful when buying an A998, because they can have either ratio in them, and in your case you want the 2.74-1.54-1.00 ratios because the low gear is 2.74/2.45= plus 12% lower..... so you can run plus 12% less hiway gear to break even, and thus help the low-C 340 get off the line/etc, as previously discussed.