I have a fairly old 600mm Sigma lens from an old 35mm pentax SLR that I want to mount on my Cannon Rebel XT, but the connection point on the lens is to small for the XT, any ideas?

There are PX to EOS mounts out there... a dig on ebay should yield some results.

Be warned however, that you'll loose any metering of light, AF, and a probable loss of Infinity focus too - so be careful!