I had a number of different bits for terminators including some left over Grey Knight heads and only the fronts of the chest, not the backs, along with the weapons, and the icon bits. The rest of the bits I had were regular legs, arms and the back parts of the chests. Here's what I did:
1.Snipped off the Storm Bolter and hand at the wrist on the right arm. I replaced that hand with the grey knight weapon/hand as seen.
2. I removed the hand and the ammo box from the bottom of the Storm-Bolter and flattened out the bottom surface with a knife and file. I glued the Bolter to the top of the Power Fist (no i wouldn't play it as a
PF) then attach the ammo box to
the bottom side of the arm.
3. For the chest, since the front of the Grey Knight doest fit right with the regular hooded Terminator chest back I had to perform some surgery on it.. I carefully used a new blade xacto knife to slice the top part off the "hood". I put that aside
then cut away some of the sides to the same level basically lower the hood it falls behind the head. I had to scoop out a semi circle into the hod top to make room for the head and the makeshift psychic hood from greenstuff. I glued the
top back onto the top. then added the greenstuff part.
4.For the Shoulders I used Scibor Shoulder pads that had very Grey Knight looking symbols like a sword and shield, wings and the large "I". Then added some icons, a greenstuff loin cloth and some press molded symbols onto the
shinguards of the legs
Depending on what you have you can order the weapons, shoulders ,icons for pretty cheap from a bits site(i use the warstore). You can also easily modify the standard terminator heads to look like Grey Knight heads with two simple cuts from a knife ( you can find that tut online). For regular terminator chests you can easily slice off the eagle wings and replace it with a simple Grek Knight icon, maybe the heraldy shield in the center of the chest.
Here's what mine looked like before paint.
