What do I add to a 4L60E rebuild kit to reliably hold 450whp?

450 hp at the wheels is at or past the top of what Speedway calls a reliable street build (about 450–500 hp — it doesn't say whether that's wheel or crank). So plan a well-built unit, not a refreshed one:

The upgrades that matter (Speedway):

• 3–4 clutch pack: stronger frictions and steels — one of the most common failure points.
• Hardened input shaft: keeps it from twisting or breaking with hard launches or sticky tires.
• Sun shell: a beefed-up shell.
• Valve body / shift kit: firmer shifts cut clutch slip and heat.
• Torque converter: stall matched to the cam and powerband; for high torque, one with an anti-ballooning plate.
• A large external cooler: heat is what kills these.
• Tuning: shift timing and line pressure calibrated for the build.

Speedway says more aggressive builds with upgraded hard parts and careful assembly can go past 600 hp, but that's past a stock-based rebuild. If you're heading higher, or the car is heavy, compare against a 4L80E swap.