Dressed weight on a domestic hog, you lose roughly 28%... Head, hide, entrails, and feet.
Typically on a wild hog that percentage is a tad higher, because they don't as a rule pack the body fat that a domestic hog does, therefore Head, hide, entrails, and feet comprise a greater percentage of their total body weight.
That critter however, according to the write up on the spearfishing website was a Bar Hog, was quite healthy, and packing lots of fat, so one would likely be closer to live weight based on the percentages for domestic hogs. Thus if you take the known factors of dressed weight and percentage of weight of carcass, you can get pretty close to live weight.
274 lbs/72 (the percentage of live weight) = 3.43 lbs. per each 1%
3.43 lbs x 28% loss = 96.04 lbs. loss
274 lbs. dressed weight + 96.04 lbs. loss = 370 lbs. give or take a few pounds.
Last time I saw a wild hog that layed across the endgate on a full size pickup like that, he weighed just shy of 390 lbs. and had nuts the size of grapefruit. But that was a long time ago. I gave him 350 before reading the rest of the thread and doing the math. He ain't your average wild pig!