Other Optiplexes did, though.
Low profile ones should, though there may be a couple that might not quite make it even so (even full size towers usually have length limits). I don't see any GT 740 SC that's low profile, only full-height. I never said all of them, only low profile.
Unless you have some other power hog in there, the PSU will have plenty of headroom. It's not like it's an FX-8150 in there for the CPU. I still haven't seen one of them fail, except for HDD or fan, myself.
1. What do other optiplexs have to do with anything? We were talking 390s.
2. Plenty of low profiles wont fit in the SFF 390s. You said "all low profiles will fit". Just isn't true. There is only 6.9 inches in clearance. There are low profile cards that are longer than that, as the other poster will attest too.
3. I have 4 390s on my work bench right now with failed PSUs. Granted that's a less than 10% fail rate (we have over 40 in the company), but still concerning to me.
4. Also with 8 gigs of ram, 1 blue ray drive, 2 1 TB drives, and a i3, combined with my 730 GT, my 11 year olds 390 is pulling 195 watts when hes gaming, give or take. So adding and additional 40w plus for a 740 or 750ti, puts it at the cap, and with no active cooling on those PSUs, just not something I personally wanted to push. Plus the 730 has handled everything he wanted to play so far.
Just my experience with them. Yours may be different.