@wargamearmathethird if it helps anything, I have few merged GameWork tech UE 4.18.3 repos that has flex too, they should build fine as is (as long as you have your dev environment setup correctly).
for 4.18.3, Blast, Flex and Flow
https://github//UnrealEngine/tree/4.18-GameWorks for Blast, Flex, Flow, HBAO+ 3, Volumetric Lighting and VXGI 1