There really is not that big of a difference. The comp.ratio, heads, are the same. The cam and rear gear ratio were different. This info. is for 1970 Impalas.
Rear gear: LS-4 2.56=1 LS-5 2.73=1 Horsepower ratings: 345 and 390
Cam shaft: Lift: .3983 I-----.4300E LS-4
.4614 I-----.4800E LS-5
The distributors were different also, and timing is 2 deg. different also.
Hope this helps.