Build up the 4L60E or swap to a 4L80E when planning a future 6.0?

It depends on how much power and what the vehicle does.

A stock 4L60E is good for about 300–360 lb-ft (≈300–400 hp in a street vehicle); a built street unit about 450–500 hp. The 4L80E is rated 440 lb-ft stock and up to 18,000 lb GVWR. Speedway says heavy vehicles and towing shorten a 4L60's life dramatically. A stock 6.0 (LQ4) already makes about 355–375 lb-ft, at or above a stock 4L60E's range — and GM paired the stronger 4L65E (380 lb-ft) with the 6.0 Vortec from 2001.

• Build the 4L60E if the vehicle is light and you'll stay within a built unit's 450–500 hp: upgrade the 3–4 clutch pack, input shaft and sun shell, add a firm valve body calibration and a big cooler.
• Go 4L80E if it tows, it's heavy, or the 6.0 will be modified.

A 4L80E swap needs:

• A flexplate with the 11.5 in converter pattern (a 4L60E flexplate is 11.062 in).
• A computer that runs it: the factory PCM it came with, or a Holley Terminator X Max (4L60/4L80 control built in).
• A crossmember and driveshaft to suit — the 4L80E is heavier and longer than a 4L60E.