From 6 to 8 syl in `66 Dart - tips?

Most of the changes you need have already been mentioned but the exhaust is the hardest part of the conversion. The only chassis headers that I've seen the fit good are the Spitfire ones but good luck on getting a pair. If you're willing to cut the inner fenders, you can use fenderwell headers from a variety of vendors. The 273 exhaust manifolds don't fit well on the bigger port 340/360 heads. If you have manual steering, you can use the '68-70 340 exhaust manifolds with some minor mods and custom front pipes.