i see several 45-70 marlin 1895 loads posted in reloading forums that will spit a jacketed 300 grain bullet out or 400 grain for that matter out at 2000fps. i was just wondering how safe these loads are? in  all my manuals iam lucky to see 1800fps out of a 300 grainer....

The modern Marlins are generally thought of as being safe to about 40,000CUP, which will give the performance you mention.

I have manuals from Sierra, Hodgdon, Hornady, Speer, and probably one or two others with this level of data.

What manuals do you have and what can you find?

Hodgdon had many loads online but may not have the "heavy" .45-70 loads on their web page.