I am thinking about rebarreling a mod 100 win. I was thinking 6.5 x 57 improved or maybe 270 x 57 improved. Would appreciate any ideas.

Bad Idea
Both of your choices are 1/2" to long for the 88-100 action and magazine . Try a 270/308 imp or a 6.5 rem imp.(6.5/308)

Rich

.260 Rem, aka 6.5-308 - you won't be disappointed

I have a Model 100 in .308, I love the rifle. It never jams, is very light and compact for a auto, I would like to have one in .260 rem, or 7mm08, or 338-08