The PLL on the T3400 is known and shared with some Optiplexes, SetFSB support is good but QX9650 runs anyway.. The T7400 (MB is all I have) has a PLL that starts with 8. There are none listed like that. Maybe one will read correctly by trial and error. I have another option there anyway, The 7400 supports 400fsb so
tape modded X5470(s) should go 4GHz.. The PLL isn't needed on the T3500 because it's an unlocked CPU, so multiplier overclocking is the method there too.
The VESA 103 issue aka Code 43 occurs because the Dell BIOS requires that resolution 800x600 16 bit color I think. AMD cards GCN3 and up don't provide it. T3400/7400 has it. Just about all other Dells too. It sure would be nice if the T3500 didn't. I have 2x R9-285s that I would like to run together. But your T7500 can display the BIOS screen with an R9-3xx card? That's great. If you go to userbenchmark.com and look up just about any Optiplex you will see only Nvidia cards because of this. You definitely dodged a bullet there. Maybe a different BIOS vendor.