Geez, I can't remember where I got it done. But it is a local shop really close to you. Like within 5 miles. They only charged me 100 bucks to shorten the rear. Call Royal Edger Mower Shop, and ask for Eric. He is a cool dude. He is how I found them. Tell him Dustin asked you to call. Should remember me, and prolly sees my dad once a week.
Not really ' Custom" axles, but new ones. To have to the originals cut, and then you are stuck with stock. And to have them cut and replined correctly is about 250 bucks. You can get some for that depending on how many splines they are. 28 splines in Summit are like 220 from superior.
It's not pointless. You are not going to have a good shot at finding one that bolts in very easily, unless your name is D.Hearne. That damn guy has all the luck. PM, tell me what you have to spend, I know a guy out there about 20 miles away that has a 9in small bearing rear from a 67 shelby. It will bolt right in.
If you want to run wheels from a newer mustang, then the wider rear axle may be a blessing. I had to use 1" spacers to get my 18" Bullet's to fit. Would have been much nicer if the rear axle was an inch wider on each side.