From the research I was able to do with part numbers, I'm 99% sure the 75 frame mounts will work on the 74 chassis. Hard to believe someone would unbolt the frame mounts, rather than pull the long 7/16 inch bolt, talk about doing it the hard way......
You're correct that the 77 design would be fairly simple to adapt to the 74 chassis, and yes you could have to drill some holes in the crossmember. I would think that finding the stamped sheetmetal that bolts to the engine might be the hardest part to come up with since there are different parts for the different chassis widths. B body, and F body look remarkably similar yet the difference in frame width makes them slightly different, been there done that one....LOL.
Luckily the factory stamped the sheet metal parts as to what chassis they fit. An A, or G body should have the correct engine mounts, but again your looking at likely only 77 as donors.