...I am installing a new 460 canton oil pa n 18-760 with the 18-761 pickup. The support bracket for the pickup appears to mount on the center main cap bolt but the mounting hole is too small?

You need this main bolt from Ford Racing, or out of a D9TE engine...or even from Canton:

Also the clearance to the bottom of the pan appears to measure about 3/4" which seems like too much.

Yes it is too much. Using proper procedures, tweak the pickup to adjust it to proper depth. Then adjust support tab to rest lightly on the main bolt's stand-off stud prior to it's being tightened into place.

Would it be ok supported off the oil pump and no center support?
Thanks

That would be unwise--the rear sump pickup tube doubles as a pump support brace and this is very beneficial in a high performance engine.

Final pickup tube set up is critical and also needs to be executed in correct and proper fashion--if you don't then you risk oil pump breakage (which is the main reason the 460 oil pump gets a bad rap....it's usually the engine builder who naively and unknowingly predisposes the load on the oil pump).  If you are not familiar with custom engine building you should probably pass this off to someone else.