Resident Big Block Guy checking in.
I have a 402 punched .30 over in my truck from a 71 Chevelle. The "400" big Block is a 402. GM made the 402 starting in 1970, and in their Big body cars started calling it a "400", but kept the 396 badging on the smaller cars. A 366 is typically a Truck motor, it came in like older 1 tons and such. It has the smame deck height, and journal sizes as the 396, 402, 427, and 454. Be aware though that GM made a tall deck 427 that was used in their really big trucks like 2.5 tons and the like. I have a stock cam from a 402 sitting in my garage, you are better off with an aftermarket. Also, be sure to replace the lifters too, as they wear together, and oiling in a Big Block is crucial, and any old worn parts might hinder it's operation. Also, I will tell you this, in my research, despite what anyone tells you, the Big Block is superior to the Small Block, just for example the oiling system has different routing that makes oil didtribution better, and will reduce bearing failure in HP applications. If you want a little gain and keep the "stock feel" I recommend the Comp Cams, 11-206-3 Dual Energy Cam, or for a bit more power the 11-204-3, both have smooth Idles and will give great dynamic compression ratios which is what you want if you want good fuel economy. Also, another thing to be aware of when buying parts, GM changed the big Blocks in 1996, so make sure that you buy for Gen IV or Gen V (1965-1996) and not for Gen VI (1996+). Let me know if you have other questions, or building/modifying concerns.
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