exhaust woes ......1973 340 into a 1965 barracuda

Since no one else brought this up-you need to address the 73 340 cast crank balance issue with the 904 The 904 was never used behind the 340, you need one of the B&M flexplates for cast crank 340 to 904. Also if your 904 is the original trans in your car, the convertor snout is smaller that the register in the end of the crank. If your 73 340 has a steel crank for any reason, disregard the above.