With the 20" barrel, I really doubt you will see 2600 FPS. Most data with the 358 Win seems to be generated with 26" barrels.
I have a Rem 600 chambered in 35 Rem. I push it hard, at 358 Win pressures and have been using the Hornady bullets. The 358 has a 5-grain powder advantage on my cartridge, but I still think barrel length might be a killer. Many of the upper loads for the 358, Hodgdon's, are compressed loads of around 122% of the usable case capacity. With the short case neck, proportionately no different than my case neck, the powder charge is near the mouth. Honestly, I don't know how they seat the bullets. What I see at the bench with this level of compression is variations in cartridge OAL. I believe it is from the lack of neck tension from the short case neck.
I've used near a dozen different powders with the Hornady bullet and have found that the best accuracy has happened to occur around the 100% case fill (with data that supports it). In my case, the 35 Rem at 358 Win pressure levels and the 18" barrel, velocities have run 2300 ~ 2325 FPS. I suspect with the extra case capacity of the actual 358 Win and 2" more barrel, you will see about 100 ~ 125 FPS more with the same bullets. I read through the referenced 25CF topic and it supports the same.
I hope you will post your results. Is your rifle a JES rebore/rechamber? I pondered those he had on GB. A one point I had briefly pondered rechambering it to 358 Win, but the rifle in its present form is too valuable to consider. Here is my 600, it is a whitetail and hog killer...