HP Z420 is dual x16 slots due to 40 lanes of processor socket 2011.
Another option is use upper x16 @ x16 slot and open end x8 slot one place above the x16 slot.
There is no alternative PSUs as they come with 600W, hence the limitation on per card watts. Around 150W or less each will work. I use standard 8x 4GB 1Rx8 unbuffered ECC at DDR3-1866. I have adapters cables in storage to convert standard ATX PSU to HP spec. I need to design and make adapter plates. Gonna pre-test to learn what are needed to keep motherboard happy since HP designed office PC and workstations to sense and control many things.
Another way is use Z420 motherboard in a Z620 chassis to use 800W PSU but finding a shell with PSU is hard enough.
I ran Z230 workstation to use Z420 600W PSU, works well, plug and play by using a home made ATX CPU 12V from 8pin to 4pin (standard pinout) short cable adapter around 6cm long, but PSU fan spin full rpm. Need to figure out what what kind of signal PSU needs to bring down PSU's fan rpm to reasonable noise. It was not PWM as I had a PWM adjustable controller at both 15% cycle and 80% duty cycle (to simulate as inverted PWM if that in case also pre-set to 25KHz).
Great Northern aka Canada.