First Savage 308 Build

I am brand new to Savage Shooter and am impressed with the knowledge base.

I am building a 308 Savage rifle. I will be using it for 200 yard punching holes in paper because my gun club is limited to this distance and occasional woodchuck hunting out to (400+) yards.
I am going to buy a Target RBLP single shot Savage action because I have it on a 22-250 and like it.
I recently ordered a Shilen large shank Savage Bull barrel with 1-12 twist rate (14 week delivery).
I would like a rifle like the 10FCP HS Precision but with a single shot target action and 26” SS barrel.
I chose this twist rate because I plan on using 125Gr Nosler BT for woodchucks and 168Gr or 150Gr Sierra match king bullets for punching holes in paper and this is what Shilen recommends for bullets 168Gr and lighter.
I would like to use a HS Precision Tactical stock (PST114) like the one on the 10FCP HS Precision because it fits me better than the stock that is supplied with the standard LRPV but I am not sure of a couple of things.

1. The HS stock with the 4.40 screw spacing is built for the varmint sized action in mat blue with the detachable box magazine. Not sure if Target action will mount properly in bedding block.

2. The standard stock is built for the thinner varmint barrel and I am not sure if it can be enlarged for the bull barrel or who could do this for me.

Two questions:

1. Should I have Shilen turn the barrel down to the varmint diameter so it will fit the standard barrel channel or will I loose accuracy as compared to the bull barrel?
I prefer the lighter varmint contour for carrying but I don't want to loose accuracy for bench use.

2. Do you know how I can neatly cover the Detachable magazine hole with a plate or if I can purchase with a solid bottom?

I will welcome any advice on barrel contour and stock selection.