I have done this! The Forgeworld kit comes with 3 sets of parts: The full plastic Leman Russ, the plastic
GW tank upgrade kit, the
FW Resin Leman Russ parts. The plastic
GW tank upgrade sprue comes with a heavy flamer with an angled mount that is very similar to the
FW resin front gun mount.
There's 2 options for what you can do: 1) Scratch-build a little plate to glue the flamer onto, so it can slide in and out of place like the
FW guns. Would be really easily with a small bit of plasticard. 2) Do what I did and cut the
FW guns from the plate they are attached to, glue that plate to the hull and magnetise the guns. Photos in spoiler..