If your bike has 126mm spacing, then it is not an eight speed bike. Eight speed spacing is 130mm, same as nine and ten speed. Do not assume anything, just measure it.
While I usually use a decent caliper for such measurement, I routinely also use a tape measure that I picked up at Big Lots for $1. It has inches on one side, and metric on the other.
Even a crappy plastic caliper will work. You can find them at Harbor Freight for $2.