I ran a heavy duty Votex twin balde 6 foot brush topper on our 1969 135, however i did keep the FEL on to balance and aid stability so you may want to stick a weight bar and weights on the front if not
Obviously worse going up hill with deck raised
I used to run in low box 2 or 3 depending on the height of the foliage / brush but could handle 2-3 foot growth in low 2nd