Can I use I-Spec B shifters with conventional clamp brakes?

What brakes? If its anything shimano since about 6-7 years ago, yes (with the right adaptors), other brands generally no although some convertors do exist.

You may need to buy a new shifter cover like:

http://www.sjscycles.co.uk/shimano-xtr-sl-m9000-cover-right-y01u98010-prod37252/

To make it work.

Brakes are XT, probably 4 years old, so might need to look for adaptors.

That shifter cover costs more than I normally pay for shifters!

A new boxed iSpec B shifter should have the right adaptor – silver barrel looking thingy with a square head. That will convert your (almost certainly) iSpec A levers to I-Spec B. Are your XTs the M785 ones (ie the ones that were current ‘styling’ until a few months ago)?

Brakes are older than I thought, M775 bought in 2011