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The only header that will give you noticable gains on an R/T is a long tube header. The best brand available is the MPx LTH... available at ModernPerformance.com.
No, you definitely dont have to drop the engine to change your exhaust header. Your going to have to remove the cat, and if you still want the cat, then have an exhaust shop cut out a small portion of the midpipe and weld the new cat back in (I would highly suggest keeping the cat). Labor for installing an LTH will probably run you $100-$200 depending on the shops hourly labor charge...