Does the ECM need a VSS signal to run the engine correctly with a TH350 (no computer trans)?

Here is what the documentation says. Each point links to its source; details depend on your exact computer, harness and transmission.

• LT1Swap's programming guidance says to keep a vehicle speed sensor when using a non-electronic trans such as TH350, TH400 or a manual trans: the PCM needs to know whether the vehicle is moving, and without the speed sensor you can suffer reduced fuel economy, reduced power and idle problems. (Lt1Swap)
• LT1Swap reports good results with the Dakota Digital SEN-01-4160, an 8000-pulses-per-mile sensor that allows cable-speedometer hookup; it will not work with drive-by-wire cruise control, for which the SEN-01-128 should be used instead, and the signal must be a sine wave (A/C voltage). The programmer can accept nearly any pulse count given the count plus tire size and gear ratio. (Lt1Swap)
• LT1Swap says it can disable automatic-transmission diagnostics when the swap runs a TH350, TH400, 700R4 or similar non-electronic transmission. (Lt1Swap)
• Dakota Digital's SEN-01-128 recreates the late-model GM 128,000-pulses-per-mile signal (equivalent to the factory 40-tooth/40-pulses-per-revolution internal sensor) for older transmissions with mechanical speedometer outlets; its 7/8-inch thread-on fitting fits TH350, TH400, 700R4 and 200-4R, with a removable weather-tight 10-foot cable (retail listings of the Dakota spec vary between sine and square wave output wording; most list sine). (Speedway Motors)

Confirm against the instructions for your exact parts before wiring. More: Transmission control in our tuning guide.