The 318 is plenty strong enough for a loader sensibly sized for the tractor. As was said above, it was a factory option. Someone who owns both can better compare - but based on basic published specs/dimensions - the 318 is built as heavy as a modern X-Series, only in a more compact bundle. (Same size front spindles for instance.)
You can still occasionally find these old Deere loaders (eBay and other places) however the price is usually rather frightening. One of the problems (other than balance) is that the 318's built-in hydraulic system isn't up to running much of a loader in terms of pressure, flow, or 'tide' handling (fluid moving into or out of the loader cylinders.) The Deere factory loader set included an auxiliary hydraulic pump (ran off the front PTO), reservoir and associated plumbing.
There were some 3rd-party loader providers (Johnson?) back then and it may be possible to adapt some newer small loader (Woods?) or build your own from several firms that offer plans.
Or - you may find 'power shovels' like the Johnny Buckets an acceptable compromise, power, lift and cost-wise.