SV in IHCspeak is Small V as opposed to the MV (medium V) 404/446, and the LV (large V) 401/478/549 and larger. Compared to the big three, while the cubic displacement wasn't nearly as large the weight of the engines were considerably higher. An SV engine weighs considerably more than a Chevy big block. This was due in large part to the much more durable and robust design of the block and the components in the block. All IHC engines have timing gears and not chains. The water jacket is considerably larger. And the webbing to support the crankshaft is immense in comparison.
Your bus has spoke wheels. They are also known as Dayton Wheels or California Wheels. The wheels are held to the spokes with wedges and lug nuts. Changing from tube type to tubeless will change the look very little. Only those of us who are confirmed bus nuts would not the difference at a distance.
Tube type tires are getting pretty few and far between. I can't even remember when I last saw an 8.25x20 or 7.5x20 tire.
At one time I owned a 1951 IHC L160 dumptruck that had tube type tires. When I needed tires I replaced the tube type with tube type tires but I had to go with 10.00x20's because that was all that was available.
Changing your 7.5x20 tires with 10R22.5 tires will give your bus a real overdrive gear. The 10R22.5 tires are at least an inch taller if not closer to two inches. That should effectively change your rear end ratio enough to give you 7-10 MPH more at the top end.