anything's possible given enough money and time, but I don't think it would be easy. I've heard of Fox parking brakes in Mustangs, but not a clutch/brake ass'y.
FWIW, here's a breakdown of the differences between '69 and '70 clutch and brake housings/brackets. It would be even worse between '68 and '70:
http://web.mac.com/jbauder/iWeb/6970clutchpedals/69-70%20clutch,%20brake%20pedals.html
When I was searching, Modern Driveline said they were planning to reproduce a clutch and brake pedal for '69 and '70. I got lucky with a guy on the Vintage Mustang Forum by putting out a want ad for some. You might try that, eBay, or wait for Modern Driveline?