Geez girl, LOL, I though you'd know all this stuff
I did this back in 73? when I swapped the 340 into the old 70 RR. NO INTERNET!!!! LOL. Dealers are no help!!!!! I of course bought 318 engine brackets, "the closest thing" that fit a 340 into a B body. I finally wised up and put the engine in on the trans mount, and then tried one, then the other. Yup had 'em reversed. AND I didn't realize until it came down to "no other choice" that you had to "space" one (2?) bolts for the 340, on the driver side.
Drove it for months, by the way, with "junkyard" supplied B body manifolds and SINGLE !!! exhaust.