I have installed 2 engines this way now in my 350F bikes, and the same would apply for a 400F as this example is a 400F eng with 350F top end into a 350F frame.
Swingarm is not installed but not essential.
Breather cover and oil filter cover removed.
From parts manual, have mounting bolts 8 x 2, nuts 22 x 2, bolt 10, spacer 2, bracket 4 (can go on later), nut 16 ready for quick access.
Lay the engine on it's right side on a couple of pieces of wood 2" and 4" thick respectively.
Lower the right side of the frame down over the engine manouvering it as required. Best standing over the engine as much as possible even if it means standing on the bike support frame as I had to.
Frame will rest pretty much in place on it's own. Position spacer 2 into position and insert bolt 10. This will assist greatly in aligning trhe position for the next 2 bolts numbers 8 (front/under/both sides).
Nip them all up a little.
Lift up on frame in such a way that everything clears the ground and stand up. Holding the steering head section higher than normal with one hand helps balance the bike, allowing the other hand to reuse those pieces of wood to support the bike. Wallah