A number of technical options:
Use the 350 ft mast as is with some different tuning to get it going on the different AM frequency, plus attempt to tune out the base capacitance of the redundant masts to eliminate parasitic effects

use a different 350 ft mast with less coupling to the others e.g. an end one

Use a diplexer to send both signals into the existing folded monopole probably quite expensive in vacuum capacitors but there may be some spares in the old BBC WS tx's and another feeder required for this option

use one of the 1296 masts.

765 would be good for flashback with regional coverage of east anglia with 1Kw