Matt:
The angle cross beam supporting the bunk supports is a good place to install guide-ons. The metal bunk supports attach to the angles, but I cannot see how in the photo. In my opinion it would not be a good idea to drill any holes through them. If the bunk supports are held on with a u-bolt which is triangular in shape to match the angle, more of these can be used to attach the guide-ons. A lot depends on your jon and the overall beam and the width between the fenders and where your bunks are located relative to the beam at the chines and gunwales. You can test it by clamping a piece of 1x2 or 2x2 long enough to reach from the cross beam to above the gunwales. If you can put a guide at an angle which clears the widest point of the beam it should work. I have done this with 1.5" x 3/16" alu angle. I had this in stock but would have rather used 1/4" alu for greater strength. I Cut 2 pieces to the proper length, and clamped one to the cross beam. I marked where the triangular u-bolt contacts the alu guide on. I centre punched, started with a 1/4" bit and then used a 7/16" bit. The triangular u-bolts I used were 3/8". This gave me some room to get the correct angle. I tested again with the bolt, then copied to the other guide-on. I then bought some 2" abs corecell sewer pipe. I rounded the guide-on top corners a bit and filed the two outer edges of the guide-on. Then I got the alu started in the 2" abs, and banged the top of the abs on the concrete floor. Gradually the alu slid down into the abs. ABS creeps in heat, so mine might loosen. The Jon should just slide through between them ok. You could use steel angle, but rust might be a problem. The beam on my Jon is widest at amidships, so the aft guide-on has to clear this.