Your torque spec will remain valid, go ahead and use (what, 21 pound-feet) the manual listing. Having done this a couple times, I can attest that it will work.
However, replacing your spring washers at the same time will not go amiss, and you can probably get new Bellevilles from the same place you get your bolts. I would like to experiment a bit to find the best configuration, but so far it would seem to be putting the "dish" washer down first (the OEM one,) then put the Belleville on with the open end OUT, and then put a flat washer down as a bearing surface for the bolt head. This will give you maximum use out of the Belleville, and still maintain spreading the clamp load (which is what the OEM dish washer is for.)
Once I come up with a better idea, I will be sure to let everyone know - but that sounds workable so far, based on what I have used Bellevilles for before.
Also, bear in mind that brass & bronze will get incrementally harder, rather than softer, due to repeated heat soaking and gradual cooling. This is opposite of the behaviour you will find in alloy steel, and another good reason to not use steel there. Therefore, the "springiness" you mention will become rather less of an issue over time, which is why I had not mentioned it before (because it eventually will become a non-problem.) If you want to restore them to their original state, next time you pull the manifold, heat the bolts to dull red and drop them in water - this "anneals" the alloy, and lets you work up the hardness curve all over again.
In short, there is no reason to change anything other than the bolts - you are assisted by the fact that this is a "low preload" fastener, and therefore installation torque is not as critical as it could be.
5-90