No problem, I put a rear 77 tank in a 70. Didn't have any trouble and I believe the side tanks are all the same size.
Did you have to remove the frame supports for the early steel tank? I didn't think the '77 plastic tank was the same shape, so wouldn't fit into the "saddles" (for lack of a better term) correctly. Not to mention that they might wear through the plastic anyway.
Does the skid plate for the '77 tank bolt to the frame, or is it held in by a strap still?
I should know that, since I've been helping with a buddies for years. But he's had an aftermarket 23 gallon for so long, I forgot how his original mounted.
Thanks.
Paul