I'm 55 years old, 6'2" with 34" arms and ride a stock Fat Bob (well, just replaced the stock seat with the new brown distressed leather <see post> from HD) and have not had any back problems -- and the back roads of Michigan suck! Maybe ease into the machine -- give yourself a few weeks to strengthen your back. Just finished 500 mile road trip over the weekend. My throttle wrist is KILLING me, but my back is fine!
Good luck -- don't give up the ride!

 

My bike started out fine, I rode it all season long. I rode my bike through fall, and pretty often in the winter. I have not had any problems until recently. I didn't have back problems until recently, and it always starts hurting at the end of the week. I will say that if I sit straight up on my bike, my arms are full extended and causes a bit of pain. I bit the bullet and got a C&C sport tour with a backrest.

Anyone know of a good place to buy longer risers?

 

i put mini floorboards on my fat bob and all the pain in my back went away

 

I'm comfortable on mine for long rides with the stock seat and bars so I bet some pull-back will do the trick for you.

 

I have the mustang wide solo with backrest and 16" apes. This made all the difference in comfort for me. Seat came first, added some comfort, but believe it or not, the apes completed it. At first I swore off apes, but after tying several sets of bars, they were by far the most comfortable.

 

There'll never be apes on my Fat Bob!

 

On longer rides I've noticed pain in my back with my '08 fat bob when it was stock, I'm 6'5" and somewhere north of 300lbs, found i had to hunch to ride, went with 16's on 2's, and scored free 3" control extensions and now I'm good to ride all day, and I like the stock seat. I found improvement in the handeling with the exception of my pegs grinding more when I lean into the twisties. bottom line, different people have different levels of comfort, in my case, my buddy had a dyna with apes, I test rode it and found it much better than my stock drag style bars so I went with apes. good luck to ya.