With the T-10 transmission, are you using a cable clutch linkage, or a hydraulic clutch linkage, or the stock Z-Bar style clutch linkage? A lot of headers will not clear the Z-Bar set up, you need to check with CJ Pony Parts on that one. Also, you forgot to tell us what cylinder heads you are going to be using. The headers you listed (Tri-Y Headers) are okay for a street car, but the primary tubes are only 1 1/2" in diameter, which may be too small for a 347 inch engine. I would say that you need 1 5/8" primaries at least....
George