I was glad to read in your post above that you will be using THHN wire in conduit.
I have seen many 10-50 and some 14-50 receptacles installed on 40 amp breakers, but invariably they were all for electric ranges, which range was rated for a 40 amp circuit, and where the receptacle is not available for someone to willy-nilly plug in a device which expects a 50 amp circuit, so I think electricians are used to installing these 50 amp receptacles on 40 amp circuits. The problem right now is that many electricians are NOT used to installing circuits for EVs and these electricians are just doing things "the way I have always done it".
Tronguy's discussion above of the history of 14-50 (and 10-50) receptacles is spot on.
I have some experience with Pushmatic breaker panels as I used to live in a house that was built in 1959 and had one. It had two columns of breakers, with a 100 amp main at the top of each column, so it is a 200 amp panel but different than today's 200 amp panels because it had two 100 main circuit breakers feeding two 100 amp busses, and you have to be careful to distribute the heavily loaded circuits to be sure they are as evenly distributed between the two busses.
These panels were actually pretty well made, and they did have one good thing: the breakers were bolt-on, not just a plug-on. Commercial and industrial panels use bolt in breakers. If you look closely you can see how they are fastened to the buss with a screw. But the problem is in the breakers themselves. Sometimes the internal mechanism gets stiff and jams in the open or closed position: and when you push a breaker to turn it on or off they will not operate, or operate very stiffly. Also, the older breakers may be thermal only.
The 240 volt buss is to the left and right of each column of breakers, and a 2 pole breaker sits between and connects to each of the busses. The 200 amp version looks like this:
View attachment 928936
There is also a 100 amp version that looks like this:
View attachment 928946
In the case of the 200 amp version, due to the fact that each buss is limited to 100 amps, I would not install a 60 amp breaker, with a 48 amp load, on the same column (with its 100 amp main) that has other large loads such as a larger size air conditioner, and definitely would not install on a column that had a 9,600 watt large electric heating load on it, which is common with heat pumps. Hopefully you have gas heat, etc.
A lot of people have replaced their Pushmatic panels, but unless you need to install a larger capacity panel, such as going from 100 amp to 200 amp, a much less expensive option is to replace any breakers that are not operating correctly with new ones that are manufactured by Connecticut Electric and are both magnetic and thermal breakers. And even better would be to replace all of the "thermal only" breakers with these newer magnetic / thermal breakers. These breakers are available on Amazon and Home Depot's web site, or directly from Connecticut Electric's web site:
Connecticut Electric has a variety of Pushmatic Breakers in stock. Browse our catalog and find the right breaker at a competitive price. Click to get started!
www.connecticut-electric.com
Connecticut Electric also makes modern breakers for other panels:
Information About Connecticut Electric
www.connecticut-electric.com
I have not seen any follow up on if you did install the Tesla Wall Connector, and did install a 60 amp Pushmatic breaker in the panel, and if you did how well it is working.
If you have not yet installed it depending on the capacity and other loads in your panel you also might consider installing a just a 40 or 50 amp circuit for your Tesla Wall Connector just to give the panel a bit lighter load so you do not run the risk of overloading it.