Might be able to get a free or very cheap Professional Products 58mm Throttle Body. My motor definetly dont need that but is there any downside in slapping it on?

 

There might be if you don't open up the plenum entrance to match it. It could cause turbulence on the flat portions stick out into the airflow which would cause it to act smaller. Otherwise I have not seen any problems.

 

58mm is too big and not necessary for a stock motor. Think about it, when you crack open the throttle, your now getting more air in with the same movement. This is going to cause the throttle to be much more senstive.

Your engine will also only be able to suck in only what air it needs. The stock TB is fine on stock cubes up to I want to say, 350hp.

Putting a 58mm TB on a 305 or 350 is pointless, will only cause worsened drivability, and is generally only necessary on a well built 383.

 

Pointless powerwise but but won't hurt anything if the plenum is ported to match.

On the bottom is my T-body I built back in 93ish. Its a stocker that I bored 2 52mm then milled the center out of. been on the car for 20yrs from a stock motor all the way 2 my 400 stroker. No problems at all.
The top one is a 58mm that I have given the TTOPspeedshop treatment to.

 

Yes their is a downside. A waste of money.

 

It will not hurt anything but you will probably not notice any performance gain unless you do other mods.

 

Unless its free.

 

still not worth it...