Because in every line up you have to have your economy engines as well.
The 307 was created by taking two small blocks and combining them.
The 307 has the bore of a 283 @ 3.875 and the stroke of a 327 @ 3.250.
It came with a single exhaust, 2 barrel only carb, small valve big chamber wheezer heads, dirt low 8.5:1 compression ratio, low ignition timing and last but not least, a set of snail high rear gears around a 2.73 ratio. The only good thing the engine got was bad, which was a 300 horsepower 327 cam. However when used with low performance parts it didn`t help matters any. The next bad thing to happen which gave 307`s a really lousy rep was the cams went flat prematurely usually taking out the rest of the engine with it.
As for the 305, it has come in some "performance" applications, but they weren`t anything to brag on. The 305 was made using the 350`s stroke, but a small bore. The bore is 3.736 inches and the stroke is 3.480 inches. This
engine came along in 1976. It was deemed a good idea at the time because chevy needed a engine that was small displacement, made enough power and met emission standards for the time and the 305 did that. Chevy also got adventurous during the later years when they tried a 262 cube small block that was a failure, then later a 267 cubic incher that was also a failure. Both were discontinued after 2 years of use.
Chevy discontinued the 307 in 1973. The 307 can be built into a excellant performance platform with the right pieces. take a 305 and a 307, put a equal amount of cash into them both, then install them in identical cars with identical gears, the 307 will blow the 305`s doors off. The small bore of the 305 hurts breathing potential. The 305 was dropped from production in 2000.
The next question, why don`t many hot rodders build 307`s and 305`s?
Why put the money into small cubic inches when you can build a 350 not only cheaper but will make more power? If I were building a daily driver street engine where fuel economy was important I would use a 305 with a factory roller cam. Add the right pieces to it with the right rear gears and it`ll deliver excellant fuel economy and good power in a V8 package. I`ll likely never build a 307, they`re hard to find now days and they didn`t come with a roller cam block and all I use is rollers. Mostly I rob the crank out of`em, have it balanced, get a 1 piece to 2 piece seal adapter and slap the crank in a Vortec 350 block.