For a long time, my Mr Fantastic was just a headswap from the SHS fig, with a repainted GI Joe Flint headcast. But I got tired of the kinda stiff SHS buck and upgraded to the MU buck, saving the good parts of the SHS...
Head : GI Joe Flint (I had a spare headcast so I repainted it)
Body : MI Captain America
Arms, feet : SHS Mr Fantastic

I also gave him a pair of extended arms so he can show his powers... It took me a long long time to figure what stuff could do the job. After many tries, I took a thin black tube, reinforced with metallic wire inside, hot-glued the two sides to stick the hand and upper arm...
The "extended" hands are Marvel Legends, he has a pair of open hands and a pair of fists. Everything is swappable.