MAP sensor is fine. Your not at sea level so the sensor is reading absolute pressure. If you get a GM shop manual it'll give the voltage vs kPA. Start the engine and measure MAP out to gnd.

If you had a wideband O2 you would be able to sort out whats going on fuel wise. Also try advancing the base timing a bit say 5* btdc.

I'd say your not far off but tuning by swapping parts is really a coarse method and may not get you to the desired end result.

Try enriching the mixture using a propane touch (un-light of coarse :)) and flow some propane done the TB if the rpm increase your lean.

51 GMC 4.2 turbo
Can't solved today's problems using the same technology/thinking that created them