so, I looked it up, because it's not hard to find that truck
popped the VIN number into the parts website and got results for allison and 4l80.
so apparently you get whatever the general was putting in trucks that day.
here's what the vin decoder says
Vehicle Accidents/Service Get Vehicle History Report NOW
Year 2001 Engine Type 8.1L V8 OHV 16V
Make Chevrolet Driveline rear-wheel drive
Model Silverado 2500HD Ground Clearance 7.50 in.
Trim Level LS Reg. Cab 2WD Front Brake Type Disc
Manufactured In UNITED STATES Rear Brake Type Disc
Body Style REGULAR CAB PICKUP 2-DR Anti-Brake System 4-Wheel ABS
Transmission 5-Speed Manual Overdrive or
5-Speed Automatic Overdrive or
4-Speed Automatic Overdrive or
6-Speed Manual Overdrive
Standard Seating 3
MPG Hwy miles/gallon Optional Seating 2
MPG City miles/gallon Tires 245/75R16
Dealer Invoice $21,360 USD Wheelbase 133.00 in.
MSRP $24,411 USD Curb Weight 4846 lbs
vin removed to make it at least somewhat difficult to fidnd it again, but seriously it took me like five minutes.
So I'd say you want to HGTD if you don't want some asshole here poaching it