Could go with telescoping pipe...but real estate looks like it is already pretty tight there. Can you go sideways with the furnace at all, that would give you more pipe length to work with. Or slide it back further, go with (2) 90's (that's assuming your chimney drafts well enough to pull that off)
Also, I wouldn't worry about using all those 6" connections on the new plenum...I'd hook it up using something larger, more of a standard install. As long as you have at least the minimum duct area spec met, should be fine. IIRC, Drolet even told someone that in an email...they said they made the plenum that way to be easy for the DIY guy to install them, doesn't hafta be done that way though...might wanna double check with them, but I'm pretty sure they said a standard plenum/duct install was OK with them.