With a 50 gallon tank I think you should have a 22-24 inch
UVB bulb. I recommend Reptisun 10.0 T5 or T8. T8 generally goes inside of the enclosure, and people usually put T5's on top because they're a lot more powerful. 8" should be a good distance for it.
https://www.amazon.com/Reptisun-260...ywords=reptisun+10.0+t5&qid=1602868817&sr=8-3 That seems like a pretty good value to me.
https://www.petmountain.com/product/zoo-med-reptisun-t5-ho-100-uvb-high-output-bulbs?variant=40630 If you want to save some money get one of those (they might be out of stock) and just a fixture with a reflector that is the right size from a home depot or Walmart.
https://www.amazon.com/Zoo-Med-Rept...words=100w+basking+bulb&qid=1602869538&sr=8-5 I use those in the summer, but my house gets colder in the winter so I just swap it out for a 150w when I feel like it's needed.
Where you place the heat lamp doesn't really matter as long as you're getting the right temps and it's not too close to hurt your bearie. I would place it on top tho.
You can mount the UVB with hooks, paperclips, velcro, or just get a T5 and place it on top but make sure it's still close enough.
By the way the thing you said you found is the
light only, not the fixture.