Got this really fast reply from Jeff at 521 Restorations about possibly using the 70-76 A Body tank.
Hi Jim,
Pretty sure you're going to run into problems with the filler neck location moving slightly between the 68-69 tanks and the 70-76 tanks. Tank size is the same. Some places like to say the CR11C will fit 68-70, but it's the filler neck location moving that messes things up when you try to use a CR11C in a '70. Pretty sure you'll have similar problems trying to go the other direction using a CR11E in a '69.
Jeff