70 Dart exhaust tips

Yes you are correct 68/69 Barracuda and 70 Swinger
have the lower valance which calls for a shorter exhaust tip and the upper hanger bracket is welded
4-1/2” from the exit end of the tip. Also if trying to install the incorrect longer tips you will find that the right tailpipe is too long as there is a compound bend
next to the fuel tank that prevents the tip from sliding forward. Like you mentioned, “Easier to get correct tips”. It can be done but why modify the tailpipes and the tip hanger bracket unless you find the challenge exhilarating and fun.