For anyone interested. The Thunderbird IRS is to wide. It was hard to measure in the position is was in but it looked to be at least 62" maybe a little more. It could be done but I don't have the skills to shorten something like that plus I'm sure It would require notching the frame rails. My Falcon is 58" so a good bit of engineering would be needed. Also the Cobra IRS is too wide. It will fit but would require fender flares for an extra 1.5" on each side. Thanks DVS Restorations for your help on the Cobra IRS measurements. Explorers are integrated into the frame so I didn't even bother to measurement one. There are after market kits available but I was hoping to do this for a lot less than $8000. Kits run from $8000-$10,000. Unless I win the lottery it looks like I will be using my straight axle.
Thank you one and all for your help,