since it is a precarved is this common to have so much work needing done?
Yes, a pre-carve is just that - perhaps 90-95% done towards the final shape as "envisioned" and possibly nowhere near the level of finish/shape you may wish to take it to yourself. Plus, each supplier has their own (definition) of pre-carve.
Although sometimes called a "kit", these are builders kits, not a "put part A into hole B kinda kits" often requiring somewhat over 100 hours of work to complete (at least the first couple you attempt), even on the pre-inlet versions.
The couple that I have have lent a hand with were not inlet at all for the tang, so the back of the barrel channel was a "rough guess" at best and in both cases we extended the channel back maybe 1/2" or so before the tang was even inlet.
To your ram rod channel - since it's drilled already I would taper the channel out towards the muzzle so you don't end up with a "step" in the channel just before the entry hole.
If the web is 1/4" at the entry and you taper it to 3/16" or even 1/8" out at the muzzle (on a full stock), over that distance, you won't notice the difference visually (unless you put a straight edge on it and reference another point on the rifle) - it will look natural and the "slight bend" will help keep some tension on the ram rod, which will keep in place better.
