Mustang GT T45 onto 5.7L SBC?

Anyone know if it's reasonable on a challenge budget to mate a T45 from a 4.6L Mustang GT to a small block Chevy?

Not reasonably. There is a way to do a T5 onto a SBC BUT you still need a Camaro or S10 input shaft and bellhousing to make it work.

3rd Gen F-Body T5's are dirt cheap and even 4th Gen LT1 T56's have dropped to Challenge-friendly levels. Those 2 would be your best bet.

As for the T45-to-SBC, I don't know of anything that would make that work or anyone who's wanted to try it before?

Now that I think of it the Ford T45 input shaft is longer than the Chevy T5 shaft (citation needed) you COULD cut an adapter plate for the appropriate thickness and adapt a steel (aftermarket) SBC T5 bell to the T45 trans drilling all the way through the T5 Bell and securing from the inside out.

See if there is a 10 spline Ford clutch disc in the same diameter as a 1990 Chevrolet Camaros, then order that disc, the Camaro pressure plate and throw out bearing.

McLeod makes a modular scattershield that will connect most engines to most transmissions

I forgot about that one Wally! They do make T5/T45/Tremec sides and obviously SBC sides so that should be an easy-cheesy bolt-in affair...

http://www.mcleodind.com/downloads/prod_guide_PDFs/Page_9.pdf