Skylark,
I can pretty much guantee that you will start on the 757. That is the core of the flying program, and as such your joining LPC, especially joining with other pilots will be on the 757. Given your previous, it MAY be after 6 months your next OPC can be on the 767 (alternating type checks required), however don't expect or assume it. There will be others, with longer time in the company than yourself, more experienced in general although just not having had 767 time, who I'm pretty sure will insist in time honoured fashion that they are not bypassed.
30W