I used 1.25" although I could have used 1.5 just as easily. 1.25 worked a little better with the perches I had. Actually you can use 1 inch if you wish. The difference between 1" and 1.5" won't actually be seen by the eye as much as you might think. 1" will let you pick up a wee bit shorter turning radius...again not enough to make much difference.

~ Jon
1952 1/2 ton with 1959 235 | T5 with 3.07 rear end