what motor matters a lot
there were 3 400ci versions in 68
base 400 D port , named for the siamese center ex ports 3 logs
a ram air version of the base motor worth 2x
and a trick rare 4 port NOT siamese center with 4 ports on the ext logs
ram airII with round port heads worth way more 3-4x then the D ports
your's looks like a base 400 motor
but rag tops also carry a premium
esp if really unrusted original with matching numbers and date codes
yes the big auction house's charge too much
but they also get the real money
and may get 20-30k more then an E-bay sale
I had a 68 goat with a 71 455ho swap in a tin top
there was a GTO restoration book that listed everything
and should be worth the hunt to find to prove everything is original
9780879389536: Pontiac GTO Restoration Guide - Paul Zazarine, Zazarine, Charles Roberts, Ph.D.