I've had a HF lift for over a year and like many others, I've added additional u-bolts but am still using the original wheel
clamp. I did add a 1" thick piece of wood to the face of the wheel clamp to act as a spacer since without it I found the clamp was only biting on the rear portion of the tire and not centered at 6 o'clock. I did make a long rectangular box (from Chinese HF crate!) so I'd have a step-up when getting the bike on the lift - engine running, trans in 1st gear and me walking the bike up.
Two improvements were a longer ramp make from left over 2 x 12 and a crossbar for the front made of 1" steel tubing. The original ramp is pretty steep so if you don't get the bike all the way up, it tries to slide back down. The 2 x 12 adds about 2-feet to the ramp and makes it a more gentle angle so if I do stop with the rear wheel on the ramp, it doesn't try to slide back down. It sits under the original ramp and has some blocks screwed to the front edge to create the angle.
The crossbar has two short perpendicular pieces of tubing that fit into channels already in the HF lift. Those channels run front-to-rear under the table and give it stiffness. Because of the angle of the tiedowns from the grips, it is not necessary to use bolts.
I've found that when doing plastic removal, it is often necessary to move the bars to the left or right so a task such as for valve adj is with the front wheel well clear of the clamp. In such a case, I use the side u-bolts to secure the bike with the centerstand legs resting on the drop-out panel. That panel is too weak to support so much weight so it bends in the center and I eventually solved that by bolting a stiffener to the underside but you could do the same thing with plywood on top. With the lift all the way down, the carriage x-braces are close to the underside of the drop-out panel so you'll have to check clearance on anything added.
I'll put a few pics of these mods in "My Gallery."
Tosh