I just helped a friend get his 73 impala running. it has a 402 bbc, th400, 2.73 gears, and weighs 4500 pounds (huge car). Cam is mild (208/212 at 0.050") 8.5:1 cr, no headers, and 26 inch tires.
It runs fairly well. spins the tires a little bit and pulls ok. feels like a 0-60 in 8 or 9 seconds (0-60 with only 1st gear). It has enough power to get out of it's own way.
3900 pounds and a 460 should run much stronger. should be ok if the cam is mild and the tires are less than 28 inches