With the huge hassle of having to re-build the motor (I did it once, top end, on the Volvo MD17C), I would throttle it back to a constant rev where it did not overheat and leave it at that.
If you have reasonable starting, and mid-range power, I would not touch it. Throttle the older motor back until it is at ease with it, and stay there.
Although 86 % of full load sounds OK for a newer motor, I would not rev the old Volvo to more than 2000 rpm for any length of time. It will take flat out for a wee while, but I don't push it, and that is with the top end rebored and valves re-cut, and collant jackets de-scaled.