It can be done, you will have to figure out what transmission you are going to use. Then get motor mounts for the 502 mount them to the cross member. Buy the Transmission cross member support and modify it to the Mustangs mounting area. Then will come rear end mating via drive shaft. The drive shaft will have to have a different yoke on the end and made to the proper length for the application... For cooling most likely electric fans and a corvette radiator will be the best. You can most likely buy springs for a big block ford and get by with spring rates on the front end, steering will be your call, but I think they make kits to install Ford rack n pinion steering on them. You will just have to mate a Chevy power steering pump to the lines for the RNP unit...
It is possible. But the biggest drawback is the steering linkage will want to go through the oil pan. (The Ford sumps are in the front.) I HAVE SEEN oil pans with a tunnel for a drag link, but I always thought that was just too extreme for the street..
I had a G altered 67 Mustang with a 427 moved back 8 inches. If I can move my front sump that far back without steering problems, I suspect if you are willing to sacrifice the firewall and build a new one like I did, then you could move the BBC back far enough to avoid the steering problems.
The only reason I see to put a BBC in a Mustang is to insult the Ford boys. There is nothing to gain with exercise but the animosity of dyed in the wool Blue Oval folks.
I would have to admit I am one.
Old Fart Ford Racer
You can just about put any engine in to any car, it all comes down to Time & MONEY.
It is strange to put a CHEV into a ford but guess they need all the help they can get. lol
Eldest boy(Died in the wool Ford Man) and me (Holden/Chev) are alway taking the pi*s out of eachother especially after annual V8 battle at Bathurst
He is currently putting a (Believe it or not) 3.8l v6 (Holden commodore) into his Toyota Van for 3 reasons, He wants to, He Can and wants something different. I think over your way this is a Buick? motor.
You must have a lot of money. Just to cut fit and weld new cross members and suspension, not to mention cut the firewall because the ignition set up is all to the rear of the motor. Then you need a transmission to fit along with new trans cross member drive shaft, u joints, exhaust that will have to be total custom. Then you will need 4 wheels disks and new brake lines suspension front and rear. Why don't you buy a Camaro instead.
The Mustang has so many hipo bolt ins from 289, 302, Boss 302, 5.0, 351C, 390, 427, 428SCJ, their is also an aluminum block 460 that is 500hp. These will bolt straight in and fit into the engine bay.
I do find Mustangs boring and a dime a dozen but I hate seeing Fords with Chevy's in them. As for the Holden in a Toyota that's great.
I added a few websites with some great options that have over 500hp Ford bolt ins.