360 internal or external balance

Thé 360 i have has a vibration above 3500 rpm. I’m running a stock cast crank (3418649)with stock 360 flywheel and damper. I’ve Noticed metal added On only 1 throw of the crank. Front throw seem to have been machined. Do it look like a crank modified for internal balance?
Also the engine is really smooth running at idle. The po told me that the engine was balanced...

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Sure looks like a balance job was done. Sounds like time to re-do, including fly wheel & damper.

Older 360's are externally balanced....fyi

Far as I know all 360's were externally balanced. However the OP's pic shows an aftermarket/machine shop balance job, not factory.

If it was internally balanced without the flywheel but had the balancer you would need use a flywheel without the 3 holes and keep the damper...Looks like it may be internally balanced with the welding, but it's a guessing game now. They should've balanced it with the flywheel...?

I just got my dad to professionally rebalance his 360. Originally we changed from stock to .030 over kb hypereutectic pistons which dropped the bob weight and didn't balance it at first. It idled smooth, but vibrated awfully over 3000rpm. I calculated the weight difference and could modify the damper weight and flywheel weight, to gain some rpm but it still vibrated badly over 4500rpm.
If you don't know the parts, you need to just have it balanced, including the damper and flywheel. I would suggest a new damper over the stock one as some cheap insurance on your crank, especially since it is a stick.
As a side note, that crescent in the side of the flywheel is a little odd. The 360 flywheels I have dealt with only have the 3 big balance holes.

Installed a new harmonic balancer, didn’t did a full test, but did a quick run to 110 mph and seems much better. Will advice.