I happen to have a lot of goex fffg powder. would this work OK in my 45/70? I know that ffg is recommended.

fffg black powder works just fine in a 45-70.

I usually use CTG, but I have used it all except 4f; when I took my Sharps 45-70 for caribou, I stuffed it full of compressed 3FG to get the most velocity I could. Worked great. So, yes, go for it.

Works just fine.
I've used a good bit of 3f in my 1873 Trapdoor because it's what I have the most of. I believe 2f might shoot a little better when I'm shooting off the bench, but not enough to matter.

I am shooting Kik 3F in my Shiloh with great results

I use 3F in every blk pwdr long gun I have !

How is the recoil in the 45-70 with BP?

Depends on weight of rifle, weight of bullet and depending on the rifle the shape of the buttplate. A 520-540 gr. bullet over 60-65 grs. of black in a light rifle with a thin, crescent buttplate can be pretty hefty. A 405 gr. bullet over the same load in a heavy rifle and shotgun style butt isn't bad at all. I've always felt the recoil impulse of black to be quite a bit different from smokeless but it can still be heavy.