CHANGING MY 727 FOR AN A518

The difference is serious, depending on your engine balance

On the older 904/727's the weight for the "off balance" engines such as a 360 are on the converter. Those cars have one "odd" hole on the flex plate

A 518 converter is always neutral balanced. If you are running it against a 360, you need an aftermarket "weighted" flex plate to put the combo in balance. A500/ A518 converters / flex plates are symettrical bolt holes, as there is no balance/ index problem. On those, the weighting is on the flex plate. HOWEVER a magnum flex plate IS WEIGHTED DIFFERENTLY than is a 360 vs 5.9

To simplify this, we need to know "what engine" you are running?