Most likely some engineer thought that the idle would be too rough with a performance cam so they put a heavy flywheel on there to keep the engine running smooth at idle.
some of the early pro-stocks and maybe s/s tried what was called a 60lb wheel. probably for launch. not a production item. I don't know the exact of production wheels but a good guess would be around 40lb.
Maybe it is a truck flywheel All the early, pre 1962, 8 bolt crankshaft don't have a centering register for the crankshaft or flex plate to ride on, they center on a machined surface on the outer edge of the crankshaft and they normally have studs, bolts in them with no threads in the crankshaft flange for the bolts to screw into I have seen that type of flywheel used in Mopar SB with small C.I., less than 400 C.I., in some of the old modified eliminator cars
Re: 58 lb. HEMI flywheel questions. - 07/11/1812:14 AM
Can you imagine a G/Gas car that weighs 3570 Lbs. + the driver weight with a 3.09 1st gear and a 7.17 ratio Dana 70 in the rear with that weight flywheel I have seen that car raced in the mid 1970s, the driver was very busy shifting (at 8600 RPM) the first 300 Ft. I drove a old 1970 Duster Pro bracket car back then (mid 1970s)that had a 30 Lb. flywheel with the 3.09 first gear ratio and a 5.57 rear gear ratio and Dana 60 out of that same G/Gas car shifting it at or near 7000 RPM with semi stock 340 motor with 13x30x15 inch slicks, I was real busy the first 300 feet also