Even if the converter clutch failed in the torque converter, the vehicle would still be under power, able to climb grades, etc. Fuel economy would suffer, but the vehicle would be driveable.
If the stator one-way clutch in the torque converter failed, you'd either have poor acceleration but normal highway operation, or normal acceleration but restricted high-speed operation. But the vehicle would still be under power, able to drive, etc.
Almost certainly NOT the torque converter. It'd be the very unusual failure that would cause your issues.
A shallow-pan filter installed on a deep pan could cause all sorts of slipping/no-power problems.
I would want a pressure gauge to tap into (multiple?) transmission pressures. That will all depend on how many pressure taps there are on the case. Some transmissions have several taps for the various pressures. But each tap costs seven cents, so GM eliminated some on later models. There should be at least ONE pressure test port.
I bought this before I found out it was made in China. Any similar gauge that reads to ~300 psi and has a flexible hose with an assortment of pipe-thread adapters to screw into the case would work.
www.amazon.com/OTC-Tools-5610-Trans-Eng-Tester/dp/B000R5G0DO/ref=sr_1_2?crid=23ESWRH3RKAZM
In addition to the pressure gauge/multiple taps, a person could "air-test" the case after removing the valve body. Apply compressed air using a rubber-tip blow gun to various passages in the trans case, listen for the sound of clutches applying (or leaking air excessively.) This can diagnose failed clutch seals, accumulator seals, servos, etc. although it doesn't provide info about the clutch/band material itself.
I expected more than one place to air-check clutches without disassembling the trans--but this video shows only one. Not sure that's accurate.
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YOU NEED A SERVICE MANUAL and what in America would be basic transmission diagnostic tools. Select service manuals may be downloaded from this web site.
There's about a thousand Youtube videos for working on the 700/4L60/'60E and related transmissions. Would be worth your time to search 'em out.