The YO Rally wheel is 15" min. at the narrowest and grows bigger at the mounting plain. I can't measure inside but it's a guess of 16.5". The 12.88" Willwood's, need less than 15", the 14"rotors need 16.25" of clearance and might fit.
When someone else has the wheel off they can give you an exact measurement. Are you really going to NEED the Big 14" rotors on an E-body? My stock >11" disc brakes can lock up. So why the extra? Looks or virility? or Road Racing at a track. I bet the 12.19 is the most the car can take advantage of, and is the optimum size for it's 3800# weight with a big block.
What is the wheel clearance on Dr.Diff's PBR 13" discs?