I have an extra GE S50 Ballast and an extra 100W HPS 201SA Bucket.  Before I start taking things apart, does anyone think it would be feasible to do a conversion.  If successful, I even have an extra yellow 25 HPS NEMA Label to stick over the 10 label.  However, this luminaire is only sold in the 50/70/100/150W Sodium varieties, so I am not sure if it is even designed to hold a 250 watt ballast.  I know back in the Merc days, they made the 201SA with 250W Merc Ballasts, but I don't think it had a capacitor, so I don't know if I need the capacitor, or if I could make it work without it.  Honestly, I have no idea what I am doing, because I have never done a conversion before, or I have never replaced a tab ballast (with connection tabs) with a ballast without them.  The GE S50 Ballast I have is all wires, so I know I will need some wire-nuts.  I just don't want to do something and not be able to fix it.  The S50 Ballast is also a multi-volt ballast (4-tabs 120-277v with 208, and 240 in between).  Does anybody else have a clue as to whether this conversion would work, or would I be better off working with a Cobra Head?  Any input or suggestions would be helpful.

The requirement for capacitor / the capacitor spec depend on the ballast, and i'd expect it to be stated on the ballast what is required

The lantern limits you in size of components and in thermal design. It is better to not cram the gear in a tight space above a hot lamp. Install it in a spacious box next to the lantern

The requirement for capacitor / the capacitor spec depend on the ballast, and i'd expect it to be stated on the ballast what is required

The lantern limits you in size of components and in thermal design. It is better to not cram the gear in a tight space above a hot lamp. Install it in a spacious box next to the lantern

Well, given the size of the S50 and the 201SA housing, It probably would not be a good idea then.  Because this project would require just that; cramming them in.  Thanks for your input

The fact the buckets were not offered above 150W is pretty simple:
The 35 to 100W and the S55 are 55V lamps, so 120V OCV is sufficient for them. So just one series inductor (with a small ignitor board) is all what is needed to run the lamp - that is the smallest and most efficient form of a magnetic gear. So these are rather easy to fit inside of the bucket head.
The higher wattages need higher OCV, so (in a 120V mains world) an autotransformer style ballasts, with HPS it means usually a CWA. And that means first larger size of the magnetic part (at least double volume and about double losses for the same power rating) plus in case of the CWA a capacitor (the CWA needs it as part of the primary function), usually about half size of the magnetic part. So we are with triple volume and double losses compare to the simple series reactor. So I doubt these would fit in any reasonable manner inside of the bucket.
What I remember from the pictures here, there are lot of 250W bucket-form lanterns, but all with remote ballast arrangement (or it is in the "power bracket" form, so the ballast is again outside of the main bucket).
With integrated ballast (above the lamp inside of the lantern head) the maximum power I have seen is a 175W MV using HX autotransformer.

Either that or a choke on 240/277V....

Since the fixture wasn't really designed for 250W HPS lamps-you might overheat the refractor.And of course as others point out room for the larger 250W HPS CWA ballast.I burned a refractor in earlier days-when I "converted" a 175W barn lights to 400WHPS and MH.The refractors melted!that was when I first turned into a flashoholic and collected lights.I used ballast kits to convert the lights-since they were inside-mounted the ballasts in a metal box by the fixture.Left the old 175 W mercury ballasts in the fixture.That was when you could get the bucket-barn lights at just about any hardware store.

Since the fixture wasn't really designed for 250W HPS lamps-you might overheat the refractor.And of course as others point out room for the larger 250W HPS CWA ballast.I burned a refractor in earlier days-when I "converted" a 175W barn lights to 400WHPS and MH.The refractors melted!that was when I first turned into a flashoholic and collected lights.I used ballast kits to convert the lights-since they were inside-mounted the ballasts in a metal box by the fixture.Left the old 175 W mercury ballasts in the fixture.That was when you could get the bucket-barn lights at just about any hardware store.

That's what I was going to do.  I have 2-S50 Ballasts made by GE.  BTW, I have an extra one.  If anyone is willing to pay for shipping, I will give it away to the first reply interested.  Not sure the capacitor is good, as it has a few dents, but that's why it's free.