How difficult is it to fit a wheel from a 2006 mustang onto a 2000
mustang, according to the specs the only difference is the offset. 
The wheels I have now have a 24mm offset, and the wheels I would like
to put on have a 45mm offset.  Wil l these bolt right on, or will I
need some kind of spacer to make them fit.

this may be of help to you

http://www.explorer4x4.com/wheels_offset_backspace.htm

it looks like your wheels will be pushed 21mm into the car. You will
need a 20-25mm spacer (depending on what you would like the final
offset to be..

you can purchase them here

http://www.livermoreperformance.com/mustang_wheel_spacers.html

or here

http://www.mustangsplus.com/Merchant2/merchant.mv?Screen=CTGY&Store_Code=MPFMP&Category_Code=wheel_spacers

just google search mustang wheel spacers and you'll get alot of results