The "72" 340 TQ is jetted perfect for a 273. I never had to make allowances. As a matter of fact they run lean but run very well and get great mpg. I would recommend the .095 primary jets and the .137 secondary jets. Don't overthink things. The "66" 273 should have higher compression than a "72" 340 and you are running the same cam. You will need new pushrods to work with 273 rockers and hydraulic lifters. How much air do you think a 340 pulls at idle? The air fuel mixture is the same on a 273 and a 340.
