Hmm it should. I run a 351 in my 65, (See Mods in sig) and I do not even use the stock manifolds. I run the Headman Mids and I have no problems at all with space. It when in without issue. See the thread PROJECT FINISHED for pics of my engine bay with he mods in my sig.
The 351 fits in the bay just fine, it is a bit tight, the only thing you would need would be some swap headers. Manifolds will be a pain to get off when you want to change to headers down the road, so if you plan on going headers at some point do it now. The bolts on the manifolds are long and the longer one will contact the shock tower when you try to take it off. I had this problem in a 69 and had to chisel the head off the bolt, so I imagine it would be a lot worse in a early stang.
The 65/66 stock manifolds will work--I did it for a year or so. They do drag against the shock towers, but not that bad. You will need a different crossover pipe because the stock one will be too narrow.
By the way, the 65/66 manifolds are terribly restrictive for such a large engine. The 69/70 manifolds definitely won't fit--I tried that too.
I don't know about the original poster, but mine came form necessity. It was 1979 and I did the conversion, not knowing what I was going to do for an exhaust. There was no internet to refer to for available options. When I did find a swap header through the available magazines, they were $400 for 1 1/2 equal length tubes (Tubular Automotive.) Can you imagine what the cost of those puny things would be in 2006 dollars? Probably in the neighborhood of $1200-1500
As a newly married college student, I saved and eventually bought them.
By the way, the 1 1/2" tube headers fit marginally and I have a much better setup now with my 1 3/4" primary Accufabs that I spent $500 for.