how many rpm's will i drop going from 3.91 to 3.55????????????????

Being old enough to have "grown up" when all these cars were new, I find all this quibbling over ratios in the threes laughable. Looking at what was actually used by the factories might provide a little perspective.
Automatics from the general would come with 3.07/.08 and 3.23 usually and strangely enough, their small motor, two speed (powerglide) combo almost always would be coupled to the 3.07. Standard ratios with sticks were usually 3.36, 3.42, and 3.73 with bigger motors tending to have the lower ratios standard. The 3.73s in my 68 Z28 felt just about right with it's 2.20 low gear. Time tends to fog the memory, but I remember the Tripower 421 in my 2+2 running about 2450 at 70 or so, and giving 16 1/2 MPG with 3.42s.
Mopar tended to be a little more conservative with 2.7sand 2.9s behind automatics (although my buddie's 66 Hemi Coronet automatic probably had 2.56s because it wouldn't really start pulling until you were doing 75 or so, about the time you were running out of road) with 2.9s and 3.23s showing up behind sticks.