Do I need a spacer to run a 700R4, and how does its length change the swap?

Yes, on most LS engines. 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. The fix is a dished flexplate (or a dual-pattern/LS-to-700R4 flexplate) with the 700R4's 10.75 in converter pattern, plus a torque converter adapter sleeve for the converter nose. Speedway sells these as kits for the 700R4, TH350, 200-4R and Powerglide.

Exception: 1999–2000 6.0 truck engines with the long crank flange. Speedway's flexplate guide says there's no easy way to run a 700R4 or 4L60E on those. The only real option is swapping in a 2001-or-newer 6.0 crankshaft.

Driveshaft length and crossmember position depend on your chassis and transmission. Measure after the engine and transmission are mounted. See transmissions for an LS swap.