The cpu E5420 exists in two different core steppings E0 & C0.
I would advise to update the board firmware bios/bmc/sdr to the latest version, preferably using the Dos Method.
Try to install each CPU in single mode operation and see if both of them works.
The board supports one or two Intel (R) Xeon Processors 5000 sequence.
Not all boards or systems support the Intel® Xeon® Processors 5400 and 5200 processors. Refer to Support for Intel® Xeon® Processors 5400 and 5200 to ensure you order the correct product.
If you are using a non-refresh board, then any 45nm CPU is not meant to be officially supported on the non-refresh board.
If your board is a non-refresh board, then only 65 nm cpu worked fine on it.
A non-refresh board will not have a letter 'R' at the end of the naming.
Example: S5000XVNSAS, S5000XVNSATA
A refresh board will have the letter 'R'
Example: S5000XVNSASR, S5000XVNSATAR
Now regarding the population for the CPU, refer to the document
If you are unsure about anything, feel free to ask.
I will need to know the pba number for the board, and sl number for both cpus.
All the best,