What is the difference between a MTD 548 and a 558?

Hi all

I have just bought an old MTD 548 from 1989 ( I think). The engine and tractor are in good condition but deck is on its last legs and I cant source a new one. I can get one for a 558 but it is a different deck - the blade arrangement is different. The 548 has a disc with 8 short blades on it and the 558 has two blades powered separately. It occurs to me that the tractor might be the same (it looks like it from the drawings I can see online) but the decks different and therefore I might be able to put the 558 deck on it. It might cost me quite a lot as I will have to buy all the parts of the deck assembly but as the mower was nearly free it would be worth it. The doulbe blade arrangement would work better for me anyway.

Is there anyone out there with a long enough memory to know the difference, if any, between these two models?

Thanks

Skinny