I know of a member on slant six dot org that did the 200r4 in a 68 Dart with a slant six. Street driver/drag car that gets close to mid 20's MPG, and runs high 12's in the 1/4 mile. This car is not just a "sunday" driver. It is a "long haul" driver.
The 200's are quite slender and from what I understand there's minimal mods needed. I think there is someone here who has modded one in. A search might find the threads.