I don't know about the 409 being superior, they both suffer from identical mechanical issues. If you can't get it done with a 348 then it doesn't matter how many cubes you add. 348 is historically first, numerically first and when you type in this web page 348 comes before 409. 700hp can be made with a 348 and few of us have that much hp in a 409 with any stroke. They made more 348's then they did 409's and the only reason people buy 409's is because they love the Beach Boys! (I have their records, cassettes, CD's, mp3 files.... )We are both happily married to very nice women, me for 45 years +.
We live about an hour apart and he has a 66 and I have a 63, superior to a 66, I have a 409, superior to a 348, I have a 4 speed, he has to deal with all these 2 pedal problems.
Other than that, I’d trade him my 409 for his 348. I’d keep the wife (haven’t met his yet so nothing personal), and the 63.
Its actually pretty cool that someone would put a 348 from a truck in a 66 SS, pretty good choice I’d say.