Just bought a new head (Marshall Mode Four), really happy with it, but cant really afford the mode four cab.

At the moment I've got a line 6 cab, stereo cab, with 8ohms and 75watts per side.

The Mode four, outputs 350watts to a mono 8ohm output,
235watts to a mono 16ohm output.

the 235 is obviously preferable, cutting out some unnecessary headroom and i'd be using all 4 speakers, not just the two.

Is it possible to rewire it as a Mono 16ohm cab? (my physics degree leads me to think yes, if its in parallel at the moment, surely wiring it in series will be 16ohm).
any advise?

google 4x12 guitar speaker wire diagram. you'll find what you need.
2002 PRS CE22
2013 G&L ASAT Deluxe
2009 Epiphone G-400 (SH-4)
Marshall JCM2000 DSL100
Krank 1980 Jr 20watt
Krank Rev 4x12 (eminence V12)
GFS Greenie/Digitech Bad Monkey
Morley Bad Horsie 2
MXR Smart Gate
I'm not too sure. I'm assuming you have 2 16 ohm speakers, since:
(1/R) = (1/16) + (1/16)
(1/R) = (1/8)
Resistance = 8 ohms

If you wire them in series:

R = 16 + 16 = 32 ohms

But don't take my word on it. I'm just a high school Physics student. Look it up.