I have a FN mauser in 7mm mag and will be rebarreling to a 6.5 Rem Mag. I have heard discussion of putting a "block" in the box, please enlighten me on this. I have another 98 mauser that has the bolt face opened and chambered in 6.5 rem mag. i see no sort of length modification of the box, feed rails were probably worked on a bit but everything else appears the same.

First order of business would be to load the magazine and see if it will feed the new cartridge. If it will don’t worry about it. If it won’t then proceed accordingly.

Just take the Mauser to your smith, tell him what you want to do and let him figure out what needs to be done and how much it will cost you.