Hi reo,
This is just my .02 but I would say you are already having doubts about converting to a 300 rear tire.
So far other than looking cool there doesn't seem to be anything positive about doing that conversion.
Look at it this way,you will be spending lots of hard earned money to make the bike handle worse,probably use more gas and make it harder to find a replacement tire on the road if you have a flat.Then you have the increased wear and tear on the drivetrain which may make the bike less reliable and will mean increased maintenance costs.
If you are doing this conversion to be stylish for others then you may be at the tail end(No pun intended) of the big tire chopper fad? Many of the motorcycle magazines are now spouting the mantra that bobbers are the latest fad because they are cheaper to make and handle better.
If that is so you may hurt the resale value of your Screamin Eagle since it will no longer be relatively stock and if the big tire craze is gone so will the demand during later sale.Unless you can hold on to the stock parts and restore it if needed.
On the other hand if you are doing this 300 tire thing because it is truly something you want and will feel like you are missing out if you don't do it then I would say go ahead and do it knowing full well the pitfalls of doing so and enjoy the bike as you want it. After all you paid for it and you are the only one that has to ride it!
If it was me and I had to have a 300 tire bike I would probably get a custom one where the frame,suspension and drivetrain were originaly built to handle the tire and all it's geometry and physics.