Bolting a 4L80E to a 2005 5.3: do I need a 6.0 flexplate with spacer, or the 5.3 flexplate with spacer?

The key is the converter bolt pattern, not the engine size:

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

A 2005 5.3 usually came with a 4L60E, so its flexplate has the 11.062 in pattern — a 4L80E converter (11.5 in) won't bolt to it. You need a flexplate with the 11.5 in pattern, the kind used on LS engines that came with a 4L80E from the factory. A 2005 engine still has the 6-bolt crank flange (8- and 9-bolt cranks started in 2009).

What Speedway's guide confirms: on an LS that came with a 4L80E from the factory, you use the stock flexplate for the 4L80E, and a spacer is used when adapting a TH400 to that factory 4L80E flexplate.

Our source doesn't spell out the spacer stack for a 4L80E on a former-4L60E 5.3, so confirm converter engagement and crank-to-converter spacing with your flexplate or converter maker before final assembly.