to order decribsing what you have what we call " corner grounded delta " service
this was pretty common some timeback but nowdays it getting rare due the safety reason and some of the electricians lack the knowage on old corner grounded delta system
unforetally you can not get 277 out of this one because there is no netural at all
I will suggest that just replace the electrical panel and get the 480/277 wye system it far more common nowdays.
please note that i am sure that most POCO will not upgrade old delta service anymore,.
my POCO regulation book qouted say it will only increase the size to match the exsting main breaker of that service it feed anymore bigger will have to convert to wye connection
and it say delta service both 480 and 240/120 4 wire delta service will be only exsting no new comustmer will get this set up at all.
I will try to find the wiring digram of corner grounded service look like so you get the idea
this is the photo of delta service i am not sure how big it will show up on your computer
This is from Mike Holt website few members expain about the corner grounded delta service as well http://forums.mikeholt.com/showthread.php?t=82707
if more question i will look for more info
Merci , Marc