Honestly Ive got some really good guys on the job here (albeit in the UK) when its finally solved I'll post it here

 

Logic would dictate that if the gas pedal is physically going to the floor something is moving it there. Obvious I know. There are only a few thing that will do that. The kick down linkage is connected to the carb. Disconnect that and try it. If it is still doing it the only other thing it can be is that the engine is moving and pulling the gas pedal to the floor. Open the hood and power brake the car. I'll bet you dollars to doughnuts that the driver side motor mount is broken and the engine lurches out of its resting position. I have seen this before but not quite this bad.

 

Both very good hypothesis. I would go with one of those. The pedal is mechanical, and is only connected at the throttle on the carb. Only other thing would be something loose near the firewall.

 

Can you post some pictures of the engine and carb? No air filter assembly? Tough to figure out blind and on another continent...

 

67T5 is right on the money. Check the trans mount while you're at it. Something is binding that linkage up and since it's mechanical... so too is the problem. I'd also take a quick look at the linkage to see if it's rubbing anything. With the taller deck height of the 351 W it is very possible that you have a resulting misalignment of the linkage. Is this a standard shift car ? If so is the clutch fork bellcrank possibly binding against the linkage ?

 

After I posted a response earlier, it occured to me... I agree with checking the engine mounts - Mustangs are notorious for breaking the mounts although I never heard of it making the car acellerate, but sure does make sense - the engine torques to the right and would pull the throttle when the mount sags. Some broken mounts lead to destroying radiators so better check those mounts quick. Hope you find the problem. We're all ears for the solution.

 

My first 67 mustang had a broken motor mount shortly after I purchased it. It scared the hell out of me at the signal when I pushed down on the gas pedal. The pedal shot to the floorboard and took off like a bat out of hell. Oil leaks on them over the years and deteriorates the rubber causing them to fail. I ended up using short pieces of chain to substitute solid motor mounts. With the broken motor mount still in place, I did the following. Bolt one link to the block where the motor mount bolts on and the other link where it bolts to the body. Works like a charm.

 

We've found what was causing it! the gas pedal linkage coming through the bulk head was catching a manifold bolt. When I was pulling away the initial torque of the engine was enough to rock the engine causing the bolt on the manifold to pull down the gas pedal, which in turn caused more to torque which in turn pulled the gas pedal to the floor & so on, only stopping when the kick down engaged. Once we realised it was this we just bent the gas pedal lever to one side so it missed the manifold bolt. Basically its cauesd by the engine being larger that the stock 289 placing the bot too near the gas pedal linkage.

It took my garage nearly 2 months to discover this!!!

 

We've found what was causing it! the gas pedal linkage coming through the bulk head was catching a manifold bolt. When I was pulling away the initial torque of the engine was enough to rock the engine causing the bolt on the manifold to pull down the gas pedal, which in turn caused more to torque which in turn pulled the gas pedal to the floor & so on, only stopping when the kick down engaged. Once we realised it was this we just bent the gas pedal lever to one side so it missed the manifold bolt. Basically its cauesd by the engine being larger that the stock 289 placing the bot too near the gas pedal linkage.

It took my garage nearly 2 months to discover this!!!

 

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We've found what was causing it! the gas pedal linkage coming through the bulk head was catching a manifold bolt. When I was pulling away the initial torque of the engine was enough to rock the engine causing the bolt on the manifold to pull down the gas pedal, which in turn caused more to torque which in turn pulled the gas pedal to the floor & so on, only stopping when the kick down engaged. Once we realised it was this we just bent the gas pedal lever to one side so it missed the manifold bolt. Basically its cauesd by the engine being larger that the stock 289 placing the bot too near the gas pedal linkage.

It took my garage nearly 2 months to discover this!!!