Indeed you must have a fixed timestep if you're doing any integration. I've always used render tweening for that. If you were using anything other than B3D, it might be a bit messy, because you have to SLERP quaternion orientations and animation can be a real pain, but B3D makes tweening easy.
Can't use tweening as my velocity is implied by current position - previous position and it'd be prone to lurching and maybe simulation explosion. Also, all my objects are part of a collection of single surfaces. I'll leave well alone I think.