From the Chevrolet Master Parts List: 1953-55 com. (exc. hvy. duty gen.) belt width 3/8", length 41-3/8", notched, part number 3713313.
Crossing this to a replacement belt commonly sold today the length would be 41-1/2".
You converted from gen. to alt.
If alt. pulley dia. and width is the same as gen., use the stock length.
If you installed a modified/conversion belt tensioner, that requires +/- belt distance, then determine what that difference is and recalculate the overall belt length accordingly.
Allow for a small amount of belt stretch.
