original 3 speed or sm420?

[Mr Big]

My 66 3/4 is down due to a sudden onset of tranny noise. I have yet to pull the original 3sp so we're not sure yet what will be involved to repair it.

Bought this truck from the original owner who kept very detailed maintenance records and the truck is currently completely original, so I'd like to stay with a factory available configuration if we convert.

Was thinking of trying to source an original sm420, shifter, and floor hump to convert to 4sp? Will these parts be difficult to find?

Thanks for the advice/help. Glen in TN.