Once you get to the City of Moresby and you happen to have a full gang and money to spare head on over to the warehouse (if you have it unlocked). It will be available via Fast Travel.
Once you are there you can buy some nails, if he has them on him at the time, then the four of you just go crazy and waste all of your money on nails and head back to the church and turn them into the guy next to the workbench.
You gain 500 xp per 3 nails separately but gain $140 back each time you turn it in so you get twice the money back and level up pretty quickly until you get to around level 25 to 30 then you can do it on your second playthrough, but instead of 500 xp it will be around 1100 xp.
There’s the tip for the day! Enjoy! 😀