It's not just the plane of focus you need to worry about. Some early dry plate holders (I have 5x7 Premo holders) have slightly larger outside dimensions than standard film holders. This screws me in two ways;

1) I can't use them in a newer camera as they are too large

and

2) Standard film holders are too small to fit properly in my 5X7 Cycle Poco #3 (I get some really nasty light leaks)

Of course your mileage may vary, but I'd try and find convertible holders (they have a removable sheath that holds film, but they can also use plates when the sheath is removed). As always. caveat emptor.