Newbe needs help MOSFETs and PCB

Greetings everyone, I am a rank amateur with electronics. I have found a Battery Isolator schematic at Discover Circuits.com and I would like to build one of these. I understand what I?m doing but not sure how to do it. I downloaded Express PCB software and designed a few PCBs. The problem I?m having is with the amp load on the PCB. This battery isolator uses P Channel MOSFETs is rated for 60 amps. My alternator is rated at 35 amps, but when I had it tested, he stated it put out 40 amps. He did not specify the voltage he tested it at. So I?ll call it 40 amps to be on the safe side. I designed a PCB where the all the components will fit I used a trace width calculator on the Internet at it?s telling me that I need trace widths of 1.3? or so for the drain and source traces at 40 amps. They are huge, and it makes sense because the traces are so thin. So in order to get very thick traces, I am thinking of using sheet copper. I figured .036? thick copper .250? wide is roughly the same cross section as #8 copper wire. Solder these to the bottom of the PCB right over the traces. Am I thinking in the right direction? How do you normally put thick copper on the bottom of PCBs? How do you normally handle large amp loads on PCBs? I am also worried about localized heat where the transistor?s pins are soldered to the copper sheet. Should I be concerned with this? I am going to put a heat sink on each MOSFET to handle the heat. Thanks for your help, Dan