Firko - use the KX A5 frame - it has excellent geometery and adding the bigger engine just makes it feel even more planted. I do not know about using a KLX frame but would guess it would fit. Rusty and Simon Healy have proved the concept with their success on them. The KDX frame is longer and does not have MX geometery, feels long on a track. Plus the KDX is a big bike when you sit on it, compared to the A5 which feels small. Although the fat thick KDX seat is a real pleasure. The engine almost drops into the A5 frame, you need revised engine mounting plates, and you have to machine down the steel collars in the rear engine mount where the swingarm bolt goes through , as the A5 has a smaller gap between the two swingarm inner pivots than the Unitrak which is larger. The 1980 KDX400A2 uses KX420 cases - these are an easier fit than the 1979 KDX400A1 which uses a bracket to connect the motor to the swingarm.
