Long range tanks in a 182 are 84 gallons, not 74. And that'll take you almost 1000 miles if you don't flog it.
With only 643 pounds of payload with 84 gallons on board, that'd put your useful around 1150. That has to be with the old 2950 MGW for the P. There's a pencil-whipped, paper STC that'll increase that to 3100 pounds with no modifications.
Trust me, your 182, in reality (not telling you to ignore book numbers here, to be clear), will far out haul that 172 if push came to shove. For the same range that 172 gives you at full tanks, you could instead put 50 gallons in your 182 and have 850 pounds payload (or 1000 pounds if you have the STC to make it legal).