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An early conehead lathe with room for a 12" chuck is going to burn the spindle bearings up if you listen to anybody who says run it at 1500rpm. I have a 1918 L&S that has a top speed of 375rpms. This is typical of a machine of this size and age. The L&S 14" would run only about 400rpm. You might be able to fudge it to 500 for limited high speed runs, but you'll overheat the bearings REALLY fast if you don't watch it. I'd set it up for NO MORE than 500rpm and less if you can. Most of these machines were designed to run on 900-1100rpm motors, so you will have to do some reduction unless you can get one this slow. Running a VFD to slow the motor down can cook the motor if you try to go too low as the fan isn't running fast enough to deal with the heat. Beware trying to use that on a regular basis. One problem is a lack of pressurized oiling to the bearings. Another is bearing diameter, they are just too large to run that fast. The countershaft bearings are probably not set up to handle a motor running much over 1100rpms, so you might fry that. Lastly, you stand a good chance of turning a 90yr old 12" chuck into a 150lb hand grenade running it at anywhere near 1500rpms. They were never made to run that fast and can very possibly sling apart due to rotational forces. So what good is a lathe with a top speed of 375rpm? Torque multiplication is a wonderful thing. Lowest speed is probably 12-15rpm. Slow enouh to really turn and cut a 12" diameter or even larger part. You can take 1/4-3/8" deep cuts using cheap High Speed tool bits all day long if the work is heavy enough to resist flexing. You can easily work down to 3/4" diam or so before it gets too slow for a good finish in steel. You can work smaller stuff, but due to the low speed, it's going to take some filing and polishing to get it slick.
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