The P35 is a decent chipset and I have it running an E4300 at around 3.1ghz. Have you checked if your 975X is compatible with an E5200?? My current 975X was not too bad for overclocking. Also do remember that an E5200 has a 12.5x multiplier so reaching 420mhz would mean something like 5ghz with your E5200 and
TBH the chip would be more of a limitation.
The P43 has PCI-E 2.0 and the newer ICH10 southbridge too when compared to the P35.
Also I would looking to add some heatsinks to the VRMs of either P43 motherboard to keep them cool as if they overheat it can also hamper overclocking.
I would look at professional and user reviews of the motherboards you have and choose the best one. Sometimes an older higher end motherboard with better build and a stable BIOS which has been discounted in price can be better than the lower end motherboards implementing a newer chipset.