Stick with 4 HD like the Large monstrous spider, but use the solifugid's ability scores (Str 17, Dex 15, Con 14, Int —, Wis 10, Cha 2 compared to the spider's Str 15, Dex 17, Con 12, Int —, Wis 10, Cha 2)?
I think that's reasonable for rend armor, though there's a real possibility of ripping someone's nice armor apart in a couple of attacks.
The stunning really lasts a long time, too. What if we reduce the duration and/or reduce to dazed instead of stunned? It could still be CR 3, though maybe the save DC is slightly weak.
I think 2d8 rounds is a typo; the original would have been 2d4, but I don't know that stunning was the same then or not. Let's either do 1d2 rounds dazed or 1 round stunned and figure out the CR from that.
I think 2d8 rounds is a typo; the original would have been 2d4, but I don't know that stunning was the same then or not. Let's either do 1d2 rounds dazed or 1 round stunned and figure out the CR from that.
The Dragon Magazine article says "Stunning lasts for 2-8 rounds, followed by 2-8 rounds of deafness" for the Hiss, so it's a misinterpretation - it's 2d4, not 2d8.
For my conversion I make it shaken & deafened for 2d4 rounds, then deafened for another 2d4 rounds.