You can take a look into to the Queing Script .
This script is using a class RepairSystemHandler, if you copy this class and it dependencies into you own script you could easly use:
_RepairSystems.SetWeldPriority(RepairSystemHandler.BlockClass.PowerBlock, 1);

The other way would be:
var setPriority = entity.GetValue<Action<int, int>>("BuildAndRepair.SetWeldPriority");
setPriority((int)blockClass, prio);


public enum BlockClass
{
AutoRepairSystem = 1,
ShipController,
Thruster,
Gyroscope,
CargoContainer,
Conveyor,
ControllableGun,
PowerBlock,
ProgrammableBlock,
Projector,
FunctionalBlock,
ProductionBlock,
Door,
ArmorBlock
}