If I haven't lost my marbles..those motherboards are of the BTX version, 95watt and 125W only cpu's (depends on this......http://www.dell.com/us/dfb/p/optiplex-740/pd)....and mount opposite of consumer motherboards..ie the mouse, keyboard, usb slots, nic slot and so on are on the Right looking at the back of the machine and consumer boards are on the LEFT looking at the back of the case.
The orientation is swapped...making it near impossible to use a regular consumer board in a proprietary case from Dell!
Your most powerful PSU is 305Watts and yes...it has proprietary mounting and connectors as well!
Dell doesn't want you to upgrade your motherboard with a consumer level motherboard...they want you to pay them for that privilege and I do mean PAY!
Sorry for that news but the facts are the facts here...Dell doesn't share!