Judging by the white a-arms, I'd guess 89 or 90.

Check the vin on the frame.  think it's the 10th letter? 

H = 87
J = 88
K = 89
L = 90

The 88+ frames are much better than the 87's but still need gussets at the weak spots.

It could have the 87 motor installed though.  Easy way to tell is to look at the jug. 6 studs = 87.  7 studs = 88+

Also there is a number on top of the cases:

m107 = 87 and 88.