904 or 727 ?

Yes a 727 will fit in your car just fine, as barracudas came with 727s ( 340 automatic equipped cars) your trans guy is correct the 727 is a beefier unit, but a 904 can be built to be strong as well, but it will cost a little more, as 904 have smaller weaker stock shafts and can break and explode under the higher stress ( torque loads) that race motors can produce. Look for a 727 from a truck/van if possible, as they usally have the bigger drums, and make great cores to build. both 727s and 904s have there weak parts, but both can be built to be great units. 904s are usally slightly quicker in a race car, due to less rotating mass, and lighter weight, so it basiclly boils down to personal preference.