I would not replace them unless they are rusty or have other obvious problems.
The size is 7/16"-14 UNC but Volvo headbolts are not normal hardware store
bolts. They are high-strength reduced-shank bolts and should be good to go.

I usually run a tap into the bolt holes to make sure they are GOOD AND CLEAN,
then brush the bolts up on a wire wheel, and install them with a little grease
on the threads and under the shoulder of the head so the whole bolt gets the
torque and it isn't taken up with friction somewhere without really getting
the clamping force. Torque in order starting at the center to 29 ft-lb,
then do it again to 58 ft-lb and then the third time to 65 ft-lb.

The last time I had a slow leak that I thought was a headgasket I found out
a few months later it was a place where the cylinder wall was thin (2130cc
big bore) and I eventually (a couple months later) ran a 5/16" hex nut through
the same cylinder (#2) and cocked the piston in the cylinder enough to split
the cylinder for most of its length. So what I THOUGHT was a gasket problem,
and for which I bought several pretty expensive gaskets, turned out to be a
big bore problem. When you have the head off you might look in the cylinders
with a black light or even get some Zyglo in there and see if you have cracks
or porosity.

BTDT - sadder and wiser!
--
George Downs Bartlesville, Heart of the USA!