The Stovebolt Site Forums The Parking Lot Big Bolts Disc brakes for 1.5, and 2 ton trucks

The 8 lug and the 5/10 lug axles are essentially same except for rotors/hub (diameter) and matching caliper bracket.

Your unusual hub centered wheels are “Motor Wheels” Not that uncommon on a GMC like yours, and some other makes of trucks too.

You say parallel front springs, which makes your truck about like an American GMC 400 series, even bigger than the 1-1/2 and 2 ton fenders right?
The steering arm attachment you describe sounds like what Spicer does, perhaps a Spicer axle/ though it doesn’t really matter who made it, it is what you have.
If your axle has same thickness at the end where the kingpin fits very likely you can swap these spindles with appropriate bushings.
However... a 3 ton truck might be a little big for P-30 front axle components, weight wise. Also, the 5/10 lug pattern on 7.25 wheel options are 19.5” (look real small) 20” (tube type) or 22.5” (hard to find, and only 38” with 9R22.5).

Check your kingpin diameter, it may be larger and allow larger spindle swap, bigger than P-30 and different better bolt pattern choices. Look in my photo album for 53 GMC which gets larger Spicer spindles and brakes with 6 lug Budd wheels.

My pictures do show 8 lug and 5/10 lug and single and dual wheels. Different stuff going on, the 8 lug and single wheel (with smaller diameter rotor) is for 3/4 and 1 ton, except troubles with overall axle width.
The 5/10 lug is best for 1.5 and 2 ton.