1974 Duster - What rear axle do I have, 7 1/4 ???

There isn’t the need to rush out and find a 8 1/4 before you drive an inch with the 340 in front of it. Is the 8 1/4 a stronger long term choice? Absolutely. However, the 7 1/4 isn’t going to blow up as soon as you move the gear selector into Drive. If you’re doing smokey burnouts at every intersection then that 7 1/4 is going fail sooner than later but it’s entirely possible to drive with a 7 1/4 and a 340.