Billysugger:
a. Consider single-sided aluminium substrate PCB material for the MOSFET stage, like http://www.bergquistcompany.com/thermal_substrates/t-clad-product-overview.htm. I can connect you with a UK PCB manufacturer who specialises in this, but you may find one in NL;
My supplier has this stuff, but it can only deliver PCB's with 1Oz copper. To handle this much current I need thicker copper.
Billysugger:
b. If you have 12V available in your system, consider an active gate drive circuit, either using discrete transistors or an integrated bottom-switch driver. I'm using the UCC27533 on my current project, (see http://www.farnell.com/datasheets/1712835.pdf); and
I have 12V available. Interesting chip.
Billysugger:
c. Consider a high-speed over-current protection circuit, with a low-value sense resistor between source and Gnd and a comparator to sense an excessive voltage drop. I'm not so familiar with the MEGA family, but on the XMEGA I configure either in-chip fault protection functions, or events (faster versions of interrupts), to shut off the gate drive if the source current exceeds a safe limit.
There are already a number of safety checks. Do you think the ACS758 running at 150Khz sampling is unsufficient? Basically I am using PWM (Arduino about 600 Hz). On average I expect the circuit to only need about 20-25A. But peak current will reach 50A max.