well first off are you sure it is a 351? bc they dont just fit into a 66. you limited on the tranny big time. and what gears you have in the rear? if you have 3.20 gears. and im gonna say maybe 120. that is an estimate. not sure you want to drive an unrestored 66 that fast either way. upgrade to a t5 and youcould hit that with no problem. and maybe lower gears lets say 2.80s. maybe more
You can use any trans behind a 351 that you can use with a 289/302/5.0. The question is not how fast it will go but are you brave enough to stay in it. Old stangs at high speed are a little scary compared to the new stuff.
It may be a 289/302 with 351 heads. That was a common swap. A 351 has a 1" higher deck height than a 302. An easy way to tell is to look at the area where the water pump mounts to the block. The top of this surface is even with the head gasket surface on a 302. A 351 has 1" between the gasket surface and the water pump mounting boss.
It does look wide enough to be a 351. It is defineatly out of a newer car. I can tell by the air conditioning compressor and I can see a ford duraspark ignition box on the left front inner fender. Looks like your car is a convertable too.
Looks good! I wish I had a car like that when I was 15. I started out with a 67 Mustang six cylinder that was smashed in the front. That was 22 years ago!
My dad had 2 stangs a hard top(65) and the Covertible(66). He passed away about 2 years ago and we just left the cars sitting. I just turned 16 and decided i need to do something about those car, cause i really do like them and love them.
Top speed for a 351 1966 stang? 
