Why? Map Sensor regulates this

Nope.

^^^Agree
The MAP tells the ECM how much air pressure/vacuum is inside the intake. The ECM uses that to increase/decrease the amount of fuel the injectors are on/off. The issue is this is based on the programed fuel tables. The ECM doesn't know that the volume if air being sucked in by the piston has increased, so it will only supply fuel based on the smaller displacement. In short the 5.7 will run lean and it will be destroyed unless the knock sensor tells the ECM to retard timing

I think we need to throttle back on the "destroyed" engine bravado. That doesn't paint a complete nor accurate picture. What you are saying regarding the fuel table operation is true, and the engine will 'likely' run leaner, but how much leaner is TBD. I bet it would start and idle fine with a good AFR. If its too lean to start you aren't going to ruin the engine. It simply won't start. IF I was the OP, you could start and idle the 350 on the stock 5.0 tune and with AFR see where it sits in the idle no load regime. It would also be extremely important to see what flow rate injectors are currently in use. From that point on, assuming you could use MEFIBurn etc, you could adjust the BPW as necessary for the wider operating envelope under different load conditions.

OP list SN as 1A307152
Which cannot be tuned with MEFIBurn it's a PCB 555 (item 3)
PCM 555