Can I replace the receptacle and breaker on a 40A circuit with 30A equivalents?
Going down to #14 is not a surprise. That is the smallest wire legal for use in any in-wall mains wiring.  This would normally be 10 AWG, but there are rare edge conditions where a smaller wire circuit is permitted to be up-breakered, and the only example I can come up with is welders with a short duty cycle.

Not being able to go to #8 is also not a surprise; the terminations are not rated for aluminum wire, and the right aluminum wire for 30A is #8.  

Anyway, here the answer is you get about 6 inches of #10 wire and pigtail.  A red wire nut, cranked down hard, should suffice for a #8 to #10 join.