I need to cut a header down on an angle to match my stairs. It needs to be cut on a 51° angle, which my circular saw can handle, but there isn't enough clearance under the joists to get my saw in there.

What kind of tool should I use to cut this? I have marked lines on the front and back, and I'm considering using a reciprocating saw and just trying to hold it steady. Is there a better way?

Easiest would have been to cut it before placing, but I really had to whack that thing to get it in and regardless it's in place now.

I need to cut a header down on an angle to match my stairs. It needs to be cut on a 51° angle, which my circular saw can handle, but there isn't enough clearance under the joists to get my saw in there. 
 

What kind of tool should I use to cut this? I have marked lines on the front and back, and I'm considering using a reciprocating saw and just trying to hold it steady. Is there a better way?

Easiest would have been to cut it before placing, but I really had to whack that thing to get it in and regardless it's in place now.

Update:

I just used a hand plane and started going to town. It worked surprisingly well.