Hot Rods Newer belt seems wider and doesnt sit in pulley good enough

Hey quick questions guys, maybe im looking at the wrong place. Trying to get my 230 dodge flathead here going. It seems like the belts the auto parts stores sell seem a bit wider than what my pulleys would like. Does anyone have any alternatives. I found an old from my fatherinlaw that sits really well and works but its old and if/whens it breaks then I be back at the same point I am now. I measure the width and I believe the old one to be a little less than 3/8ths and the new belt seems to be around 1/2inch. But clearly says its a 3/8th belt.

 

Go to the Gates belt site. The old belts have different widths and angles on the sides

 

You might try looking at Restoration Supply.

 

The only other belts I know of are FHP belts. Fractional Horsepower.
They are common now for riding lawn mowers and garden equipment. Napa has them.