Are 340’s rare

Mean416 said
A 340 short block, equivalently built, would probably be a $500-1000 premium compared to a 318 or 360 based engine. The $2k is overblown.

Ironically a 273 probably would cost more because of the specialty pistons required.

$227 difference between Egge L2222 for 273 and KB243's for 340. Everything else should be close to the same. (rings, bearings, and gaskets)
Machine work is the same. Your 273 might cost less up front. 273's are cheap to free. 340's are $200-$1000 or more.