They fit. You will need 1 3" JR servo extention for the front elevator servo. JR servo leads on the small servos are shorter than the E-Flite 75's or Spektrum 75's...so the 1 extension is required.
ALSO you will have to find servo screws. The stock screws are too short and the heads are not large enough to hold the JR servos in place. If you use the hardware that comes with the JR servos you will likely have issues as they are MUCH larger in diameter. So I'd look around for longer screws but keep the diameter close to the stock ones. You may have to put a washer under the head too as the JR servos have a much more open area where the screw passes thru.
I have installed a set of JR 285MG's in my modified Blade400. I did not use the gromets or brass eyes that come with the JR servos. Just bolted them in place. And measure the JR white servo horns to match the ball locations the stock Blade400 horns come with. You will have to put the balls on the INNER side of the horns too when you use the JR servos. This is due to the servos sitting PROUD in the frames vs. the stock servos. No issues...just make sure your linkages run true with the least amount of angle.
If you do that it will work easily.
I've got 2 B400's. 1 totally stock and 1 modified with the JR stuff. BOTH fly great! The stock 400 is quite the bargan. It flies very well as is. But I know how guys like to UPGRADE. <G> I love how this heli finds a groove in the air.
aka - GMRO