koditten, thanks for the pic; found this thread on a search. I'm doing the same thing, cap shopping, and don't want to pay new prices. Silverado, 2001 should be new body style, extended cab / 6'5' bed, I'm guessing 2000-2007 at a minimum, possibly through 2013 for "same" size.
I did some looking on the web, and found the following dimensions. First, the Tundra, for reference. I measured the bed on mine, next to the cab, and got about 70", and a 78" length, and IIRC 21 or 22" cab height. The link I found this at was:
Tundra, 07-11, DC, 79.25" long, 71.25F/69R
I measured a little different on my truck, need to double check that I guess (measured last weekend). Moving on, here are the measurements for some other trucks:
F150, 04-08, RC, EC, 80.5" long, 68F/66.5R
F150, 97-03, 80 long, 70.5F/69.25R
Dodge, 02-08, 78.5 long, 71.5F/68R
Dodge, 94-01, 80 long, 71.5F/68R
Dodge, 72-93, 79 long, 71 wide
Chevy, 01-06, 81.5 long, 71.5F/68R
Chevy, 88-98, 80 long, 69.75F/67.75R
Chevy, 67-87, 80 long, 71 wide
F250 appears to be different bed size than F150; I asked one CL seller, as I *thought* F250 was close to F150, and he measured 84" long, 75F/72R, 25 cab height. I'm guessing 3/4tons are generally different than 1/2ton. I listed only beds around 6.5' length, as I was only collecting data for my DC search. I would ask any CL seller to measure the cap base width/length and cap height, and confirm with your measurements, so as to save a trip. YMMV, and good luck.