just bought a "long recurve" 55# @ 28"
A freind of mine gave me some Cabela's Carbon Hunter arrows 55/70. The box had a chart and in my bow's section it says
"400 55/70" So I've got the 55/70 part covered but what is the 400 for?

55/70 is to say its an arrow that could be used for a 55-70lb compound. Thats of course depending on draw length, and arrow length and tip wieght, etc. The 400 is the spine or .400" spine that goes along with that.

If you are shooting off the shelf, .4" spine is going to be a little heavy. You would probably want .45" or something like a 2018 aluminum. If shooting a rest, then .4" is probably good depending on your draw.

I agree with BC.. I have experimented with various arrows for my recurve because I shoot 220 grain broad heads. I've found the 400 spine work the best for me even though I'm a tad underspined, I have about the same set up as you.

Alright. Thanks for your help guys