Convert Bushmaster Dissipator to 6.8SPC

[kazager]

I bought a Bushmaster Dissipator, 16" barrel with carbine gas system and a2 handguards with a2 front sight block. I'd like to get some ideas on how you guys would do this. I am strongly thinking to try to keep the gun as is with only a barrel/bolt change but I do not know how to be sure that I get a 6.8 barrel with the correct profile (considering there is a gas block & a front sight staked to the barrel. If anyone knows which barrel has the right profile for it, that would be VERY helpful. I might add on a free float tube, but guess I might have to modify that myself.
Second thought on the same upper is to go with a 16" barrel with carbine gas block and put a railed handguard on it, the question is "who makes a handguard with a rail that butts right up to the a3 upper rail?
Pics or links could also help, shooting an AR is not new to me, but customizing them is.

[Iowa Hick]

Honestly if you are considering a new barrel, bolt, and handgaurd, then I would just build/buy a complete upper. Then you would have the option of 6.8 or the cheaper .223.