Why is my 98 sierra backfiring and burning double the fuel

"reckon you may have fried or gas fouled the spark plugs at those two cylinders you had crossed? if so, those cylinders still may not be firing, meaning your power is down by 25%, and might also be dumping raw fuel into catalytic converter, impeding exhaust, etc., at the same time.

replace those 2 plugs first, i would guess."