I was just running something my head the last few days and they classify the 260-289 as a Windsor engine and that got me to thinking if a 351 W would fit in a first generation Mustang without major modifications?
My hat in the ring here....
Yes- it fits
No- it wasn't a pita (tighter fit sure, but no more difficult than you make it yourself, right?)
Yes- you should use adjustable motor mounts for clearance
Yes- JBA shorty headers fit just fine (on mine at least)
Yes- plug to shock towers is close, but still manageable with proper tools
Yes- it's a ton of fun to drive and I'd gladly do it again
All the above comments are based solely on my experience of building, installing, driving, and enjoying the aforementioned combination. Any statements made here within are not to be construed as internet law, mass consensus, or defamatory to any individuals that may have done it differently.