Other V8 Regals from the G-body era will either have a Olds 307 or a Chevy 305. Neither of which you want.
Get your hands on a Buick 350. They were produced from 1968-1980.
Direct swap meaning it will drop right in on the 3.8 V6 mounts, and all the accessories from the front of the V6 will bolt right up. This saves you considerable cost over putting an Olds or Chevy in it, which would require you to change mounts and swap all of the bracketry to make it work.
Get your hands on a Buick 350. They were produced from 1968-1980.
Direct swap meaning it will drop right in on the 3.8 V6 mounts, and all the accessories from the front of the V6 will bolt right up. This saves you considerable cost over putting an Olds or Chevy in it, which would require you to change mounts and swap all of the bracketry to make it work.