Who says they're 65A MOSFETs? The manufacturer? Or you? If it's the manufacturer, sit down and calculate the switching and conduction losses and find out the temperature rise of the FET- it's probably a lot less than what they say it is. My 75V so-called 160A MOSFETs can only handle 10A without no heatsinking, and with passive heatsinking they can only handle 25A, and active air cooling heatsink can handle 40A- that's with 10kHz switching. Those amp ratings don't tell you much- you should manually figure it out from the RDon.
Are you sure you can switch that many paralleled MOSFETs at that frequency? It sounds like you might run into problems providing enough gate drive current (unless you already sat down and calculated those out too), or you might run into problems with getting all the FETs to turn on at around the same time. 15 FETs is a pretty long chain.
If you use FETs like this, you could just bolt the damn things onto a metal plate, and use wire and forego the PCB for all the power altogether.
<looking for the link, IRF changed added a new product and it has screwed up the link on their website, it's a big flat package with screw mounting tabs and 4 screws on top for ring terminals>