Will my early-90s Tahoe/Suburban TH350/TH400 or 4L60E bolt up to an LS without grenading, and what adapters are needed?

All three can bolt to an LS — they share the GM bellhousing pattern, give or take a couple of bolts — but each needs the right flexplate/converter setup:

Most LS engines have a crank flange that sits about 0.400 in shorter than older GM engines relative to the bellhousing, even though the bellhousing bolt pattern is nearly the same. Older transmissions and their converters were built for the old position.

TH350: a dished flexplate (or dual-pattern flexplate) with the 10.75 in converter pattern, plus a converter adapter sleeve.

TH400: its converter uses the 11.5 in pattern, the same as an LS that came with a 4L80E. If your LS came with a 4L80E, space the factory flexplate out with an adapter. Otherwise use a flexplate with the 11.5 in pattern plus the spacing solution for your crank.

4L60E: LS 4L60E flexplates use the 11.062 in pattern. An early-90s truck 4L60E isn't an LS-spec unit: our transmission data notes LS versions use a 300 mm input shaft and an LS converter (non-LS units are 298 mm), so plan on the LS-correct converter.

Converter bolt patterns:

Transmission	Converter bolt pattern
4L60E, 4L65E, 4L70E	11.062 in (sometimes listed as 11 or 11.08 in)
4L80E, TH400	11.5 in
Powerglide, TH350, 700R4	10.75 in

Also note that 2009+ LS engines use 8-bolt (or 9-bolt on the LS9) crank flanges, which need their own flexplates. Electronic transmissions (4L60E) also need a controller — see transmissions.