risky. It is not as simple as you think.  a mistake somewhere could damage MB or psu or both. unless you are very experienced w psu adapter it is better to use oem psu.  3910 uses proprietary 8 pin +4 pin for MB. for your interest, this is a reference in progress [URL]
Hello everyone! I was given a Dell Inspiron 3910 computer for Christmas. I wish to replace the Dell PSU with a small form factor, fully modular PSU. If I measure the voltage of each pin of the Dell PSU cables and the voltage of each pin of the modular PSU, can I make my own adapter cable? I appreciate any help anyone can give me on this topic.  
Custom PSU adapter cable