How much power does a stock 5.3L make in an E30 swap?

Stock 5.3 ratings depend on the version:

Engine	Size	Years	Power	Torque	Compression	Block	AFM
LM7	5.3 L	1999–2007	270–295 hp	315–335 lb-ft	9.49:1	iron	No
L59	5.3 L	2002–2007	285–295 hp	320–335 lb-ft	9.50:1	iron	No
L33	5.3 L	2005–2007	310 hp	335 lb-ft	10.00:1	aluminum	No
LY5	5.3 L	2007–2009	315–320 hp	335–340 lb-ft	9.95:1	iron	Yes
LMG	5.3 L	2007–2014	315–320 hp	335–340 lb-ft	9.60:1	iron	Yes
LH6	5.3 L	2005–2009	300–315 hp	330–338 lb-ft	9.95:1	aluminum	Yes
LC9	5.3 L	2007–2014	315–320 hp	335 lb-ft	9.60:1 or 9.95:1	aluminum	Yes

Speedway's rule of thumb: Stock 5.3 is rated around 320 hp / 340 lb-ft; stock 6.0 around 300 hp / 360 lb-ft, depending on version and year (Speedway generalization). These are GM's crank ratings, not wheel horsepower. The AFM engines (LY5, LMG, LH6, LC9) need an AFM delete if you plan an aftermarket cam: Gen IV AFM engines need an AFM delete to run aftermarket cams; many also delete VVT, which requires a Gen 3-style timing cover in place of the VVT cover with integrated phaser.