large tiles over plank subfloor

Hello, I have a home in France built in 1907 and am planing on tiling a second flevel floor. House built of stone. Floor joists are 3 inches wide by 8 inches deep , 16 inch centers.
The planks overtop are two by fours. Flat, but some are cracked and some have been made narrowers to fill spaces.
Should I lay 3/4 inch plywood over the planks or should I remove them.
Keeping in mind that some of them go under the walls of adjoining rooms.
I have 5.5 cm of height to reach where the old terre cuit and old cement layer were.
Thank you