dell optiplex 755 new case?

it is not a atx standard mainboard, nor a mini itx mainboard either

there is nothing in it that can be modified to work in a case it will not fit into, you can't cut or resolder anything in it

what you need is a case that will work with it

something like this perhaps?

btx case

http://www.ebay.com/sch/BTX-Computer-Cases/42014/bn_5858369/i.html

not sure if it can fit that board in there

any reason to play with that board? too cheap and has cpu and ram with it, perhaps add a gpu and make it a small gaming pc?

a low profile gtx1050 and 8 gbs of ram perhaps?

if the btx doesn't fit, you could consider making a custom case

https://www.youtube.com/watch?v=iB_69q2no34
 
You could make it fit in an atx case with serious modding. Cut the back of the case out and drill some new holes for standoffs. Or cut the btx case to fit it in like that link although that's converting a btx case to use an atx mobo. It's a similar concept. You might as well just keep the dell case. Any reason to switch?

Optiplex 755 are also very cheap. $50 on ebay for a whole pc. That's a typical price for a core 2 quad pc.