You can buy new screws for your drums that use an Allen wrench (hex ). They are called socket cap screw. 3/8 x 3/4. Also if the F6 is the same as F5 you can get new front and rear seals from Joblot Auto in New York. The rear # is BB 1175 c @ $14.95 each.
They are stepped seal held in with a retainer lock ring. Joblot seams to have a lot of parts for these bigger old trucks. I am working on a 1951 M5.
The screws are available from McMaster Carr box of 10 for $7 plus shipping.
I double checked my invoice and that is the right part number BB-1175-C for the rear seal and 91T-1190 for the front seal @ $9.95. They list it to fit from 1934 to 1952 for rear and 1939 to 1952 for front seal. I purchased them in March of this year. Ralph is very knowledgeable about what they sell and our trucks at Joblot.
The chisel trick Gotanoldtruck talked about worked for me also, by grinding the edges down so they fit flat against the slot of the screw unlike the tapered screwdriver seemed to allow more torque to get them out. And lots of soaking with spray.