IH used a special brass "nut" with a ferrule attached that separated the first time it was tightened.
It's unlikely you will find the correct part locally except at the dealership (IF they still stock them).
This is the special fuel line nut with attached compression sleeve IH typically used, current part # 222-503.
If this is what your tractor originally had, the sediment bowl and the carb inlet fitting are made to accept this special nut.
LOTS of sources, from the FIAT dealer to IH restoration parts sources on the 'net, GOOGLE 222-503. They are not real expensive, not a big deal to get the correct part and do the job right