Was wondering if there is a huge difference in hulls. The reason why is I was going to buy a Myco trailer that had an older 41' Formula on it. I've got an older 42' Fountain. Will my boat sit on this different trailer??

You're always better off with a trailer built for and / or adjusted to a specific hull. So the thing you need to look into is whether or not the trailer can be adjusted to fit your Fountain - which is a different hull design. Remember, you can do a good deal of damage to both the trailer and your hull if they're not matched up. We're talking well over 10,000 pounds here.

They are different bottom designs. The Fountain has a pad on the bottom, the formula does not. So whatever supports the keel will be different, along with the location of the lifting strakes. You want your trailer to support the boat properly. Chances are, if its a custom built steel trailer for a 41 Formula, you'd probably have to make extensive changes to it to support the fountain properly.