Can I run a drive-by-wire LS with an older non-electronic transmission like a TH400, and where does the VSS go?

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

Throttle (drive-by-wire)

• E67; LSA; 2009-2013: PSI customizes its 58x harnesses to the customer's selected accelerator pedal, alternator, VVT camshaft, intake manifold, and MAF sensor before shipment; using incorrect components means the engine will not work correctly. (PSI Conversion (via manuals.plus))
• E67; LSA; T56, TH350, TH400, Powerglide, 700R4; 2009-2013: PSI recommends using a VSS with a T56 or non-electronic automatic (TH350, TH400, Powerglide, 700R4) on its 58x DBW harnesses; failure to use a VSS can cause unexpected stalling during hard braking or an inoperable throttle body. (PSI Conversion (via manuals.plus))
• E67, 0411, 242; TH400: On a DBW swap behind a TH400, PSI recommends fitting a VSS; without one the controller can stall the engine during hard braking or render the throttle body inoperable. (PSI Conversion (via manuals.plus))

Transmission control

• 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: Throttle (drive-by-wire) in our tuning guide.