I did several conversions with another method. I bought at the auction site some newer versions of the Lionel F-3s which came factory-equipped with TMCC and RailSounds (these versions also have constant voltage and directional lighting, electrocouplers, finished cabs with crew figures, etc.), and I swapped the shells with the older F-3 shells I wanted to keep and use. I then took the shells that I didn't want from the newer engines, and attached those to the older conventional chassis', and sold those engines.
Swapping the A units is a snap; it only requires the attachment of the TMCC antenna to the roof of the older shell. The B unit with the RailSounds unit and speaker takes a bit more work. I've done this two ways. The newer B units with RS have a circular plastic rim molded inside the roof of the shell, which the speaker bracket attaches to. One method I've used is to cut out a rectangular piece in the top of the newer B unit roof around the rim with a Dremel, and then glue that piece inside the roof of the older B unit I will be using, then re-attach the speaker bracket and speaker. This goes relatively quick, but the newer B unit shell will be unusable after the surgery, of course.
The other method I've used is to find a circular piece of plastic for the rim, cut the bottom of the new rim to fit the inside of the shell (which is curved somewhat), and glue it in place. This also requires cutting and gluing four small plastic pieces spaced around the rim into which the screws are driven to hold the speaker bracket in place. After a little searching, I found a perfect sized circular plastic piece to use for the rim (being same diameter as the speaker) which was a lid on a liquid soap jug sold at a dollar store. Using this method is a little more time-consuming, but preserves the unused B unit shell.
The result is an A-B-A F-3 set with the older shells, but fully equipped with all the features of the latest TMCC/RailSounds-equipped Pullmor F-3s. Incidentally, these sets also have TMCC dummy A units, which have directional lighting and TMCC-controlled electrocouplers.
