if your problem is ONLY a cracked exhaust manifold...it shouldn't have any effect on burning oil...
a crack in the exhaust, depending on its location, can cause some funny stuff though...the system relies on pulse vacuum to help pull burned gas out of the chambers...a leak will ruin that vacuum, causing something known as exhaust reversion...in which pressure builds up in the wrong direction around the crack...the cylinders will 'push' the burned gas out, but the pressure gradient will stall it and try to push the exhaust gas back in...causing poor driveability, ridiculous idle, and hesitation in various rpm ranges...
depending on where the crack is...it can also interfere with the primary o2 sensor readings...making the engine mismatch fuel requirements in light load conditions and idle...making the problem even worse...
the bigger problem though...is possibly what caused it...I'm not sure on 98 models...but 99+ model ecu's only flash the ecu for sustained mis fire conditions...so its a p0300-304 code, meaning its misfiring non-stop...a mis fire will cause unburned gasoline to get inside of a hot exhaust pipe...in which it'll ignite there...and quickly burn up the manifold and/or catalysts...
first...fix the crack...replace the part, have it welded, whatever...while doing this, see if the catalysts check out...chances are they are now dead too; at least if you continued trying to drive with a constant misfire condition...if after the system is in check...you still get misfire problems...stop driving, you're just going to recreate the problem again...and i'd look into coils, plugs, wires, etc...to see what exactly is causing the misfire...
you may have leaking valves, or cylinder blow by problems that are causing the oil burning problem...and THAT may be whats causing the sustained misfire, of which eventually burned up your exhaust system...a crack by itself shouldn't blink the ecu constantly, and shouldn't do anything to oil consumption...it'll just drive like shat until you get it fixed, and may throw a solid code from time to time...but flashing...you have a lot going on it seems...