design for a PC/104 interface bus expansion
B19 on the PC104 does no such thing. It is RFSH*, and is used as part of a refresh cycle for dynamic RAM. You need a decoder for A4 - A7 to do what you want.

A 373 will do exactly the opposite of what you want, since it accepts data when LEN is high, and 138 outputs are active low. Use 374s instead. 

EDIT - And, of course, I forgot that the ISA bus uses 10-bit IO addressing, which was then carried over to PC104, so a decoder (not needed as long as you stick with this board) will need to operate on A4 - A9.