As noted, excessive brake pedal travel is not a problem that a proportioning valve is going to fix. In addition to the rear shoe adjustment problem noted by Ron, you should also make sure that the master cylinder does not have leakage issues and that you do not have air in the hydraulic system.
You would typically add a proportioning valve to alter the brake balance. If you are getting premature lock up of the back brakes a proportioning valve may help. I assume that you are not talking about fitting one of the brake pressure control devices that Volvo used because they are set up for the split circuit system which would have a single rear disc on each valve so would have an incorrect balance for what you want. They also appear to be on permanent back order.
Most modern brake balance vales appear to be dual circuit; but, if you search around you can find single circuit valves. You want one which allows you to adjust the pressure to the rear brakes. Not all of them allow this - some come pre set with a presumptive balance for a disc front, drum rear set up. This Wilwood unit is single circuit and appears adjustable; but, it does not appear to have any locking provision for the adjustment knob which would be a non starter for me. Shop around and you can find lots to pick from
https://www.summitracing.com/parts/wil-260-8419
On the installations that I am familiar with the valve is usually mounted in the engine compartment directly in the line to the rear brakes. This allows for moderately easy access to make balance adjustments. 'Real Racers' will mount the valve inside the vehicle at a point that is easily accessible so they can make adjustments on the go. Any location that is in-line with the brake line going to the rear brakes will be equally functional.
Adjustment of the valve will depend on what you want. You are pretty much going to have to find a section of road and repeatedly jam the brakes on to test for lock up and then adjust appropriately. Make sure that the rest of the brake system is in good condition and that the tires are fresh before you test. Do the set up with old slippery tires and the balance will change when you install sticky new tires. It will also change if you change to different brake pads or shoes with a different friction profile.
If the car exabits severe pulling to one side under heavy braking, you may have a wheel alignment problem or more likely a problem with one of the front brakes which could be a caliper problem or pads (got careless and got grease on a pad?). A proportioning valve will most definitely not fix a problem with the car pulling to the side under heavy braking. Time to inspect the front brakes.
Personally, unless this is a competition car I would not do this. Make sure the tires are good and the brake system is fresh and drive prudently (this is not a modern car with fat tires and ABS). Done incorrectly you could really screw up your brake system.