Bondi wasn't packed in that photo above. The crowds there can be 3 times what you saw. The northern beaches are quite pleasant and I like the drive up the peninsula in what is surely a subtropical paradise.
I am modifying a Nautilus double chest machine. I got rid of the pec deck function and converted the decline part to cable drive. Talk about a big job. Plus I added some features Arthur never thought about. Like heavier plates on the stack and a handle on the lever to do triceps work on. That is a job I don't care to do again. I have another Nautilus duo chest machine but I can't see myself modifying it. I so hate those %#@%@ chains. That is something Arthur used on machines that gym owners will always hate. Those chains are dirty, they stretch and they can break. Plus they are just awful to service and replace. I will post a couple of photos of the finished machine soon. The plastic foot pad broke so I am using a 10 mm solid steel plate covered with 6 mm aluminium. Should look great and this machine should survive anything thrown at it. Would you believe the lads sheared three 3/8 inch bolts on the negative cam!! I have replaced those with three 1/2 inch high tensile bolts. If any human can shear those bolts he can take the machine home with him.
Vince, if you're serious about your mods to the machines why are you using 10mm steel + 6 mm ali for the foot piece? The cost of buying them individually, joining by welding or tapping for bolts, the time required... surely a 16mm piece of ali treadplate would be strong enough? If not, use stainless.
If converting from a chain to a cable is a pain, look around for new chains. The stretch you mention is a design flaw which has almost certainly been corrected by modern day chain makers. On the other hand, replacing sprockets with pullys can't be that hard, is it? I'm unfamiliar with the machines tho, so who knows... Maybe just adding a tensioner bolt at one end of the chain or the other which you could crank down if the chain stretched would be easiest.
What's a negative cam? Is this something which needs to rotate, and the bolts are the pivot points, one in each piece? If so, using a bushing might help, or a stainless dowel pin (they come threaded one end as needed). Sounds like you got this one sorted tho.