BOSS 429

I'm pretty certain the shock towers were physically moved outboard along with the lower suspension mounting points. Boss 429s have a wider front track than other 69 & 70s mustangs. Kar Kraft of Brighton, Mi. did the modifications to the cars back in the day. I've not seen any specific details in writing of what all was involved. I would check around the net for a Boss 429 forum or registry and ask around.
 
This is what I ound on a quick search


Kar Kraft faced considerable obstacles in the development of the Boss 429 Mustang, including the fact that the engine bay was over 2 inches too narrow for the motor! This was solved by moving the entire suspension out a full inch on each side. This required new reshaped shock towers, widened strut tower brace, redesigned top and bottom suspension arms, redesigned heavy duty uprights, and heavy duty springs. Also a smaller new power brake booster was needed to clear the big engine valve cover. The battery was placed in the trunk to improve the cars weight distribution; also the cars rear suspension was beefed up with staggered shocks, heavy duty springs, and a thick rear stabilizer bar. The car sat an inch lower overall than a stock Mustang did, so a special "short" front spoiler was designed to fit the clearance available.