53mm is too much for a 318. Stick with the 50mm. Putting on too large of a throttle body will give the same results as over caburating a carburated engine, loss of bottom end.

 

53mm TB would be for a fairly heavily worked 360 or bigger.
50mm is good for a mild to moderate 318.
I have a 52mm on my 318, and between that and the FI Airgap, I did loose bottom end... however, after fitting LT Headers, and HemiFever's SCT 93 Tune, I have it back, plus a little... but with a modified kegger and 50mm Fastman TB, I would likely have more bottom end at this point... 52mm is really for a mild to moderate 360.

PS; Most aftermarket items are designed for the 360/5.9, not the 318/5.2, so us guys with less cubes have to make do.

 

I think the 50 is pretty darn good until you start doing serious mods. Meaner cam, bigger valve heads, better intake...... headers, etc. Even so, the larger TB is meant more for high rpm applications..... you want VELOCITY, not volume for a torque motor.

 

^^^ +1 I have a 52mm BBK tb here tried it and even with my mods I lost some bottom to mid range tq, mid to upper rpm was amazing with it mind you. I still run the stock unmodded tb.