Im new to this too so don't worry. the 327 had a longer stroke that will essentially give you more power per stroke. But because the piston travels farther you need a piston with a shorter pin to crown height so the pistons don't come out of the top of the block ( 307's and 327's had the same stroke and essentially the same pin-crown height). The only reason the 283 block is used in this case is because the 283 from the late 50's - early 60's had thick cylinder walls that can be bored out to give you a greater diameter cylinder, again adding to your power. Increasing the 283's cylinders like this (0.060 over) gives you a displacement of about 302 or around there i think