Converting Imperial Nut Brown kit to Partial mash

I would like to try to convert some of the LME to a partial mash on this Brewer's Best - Imperial Nut Brown kit.

5 gal, OG =1.071-1.075, FG = 1.016-1.019

6.6 lb Amber LME
2 lb Amber DME
8 oz Maltodextrin

4 oz Caramel 80L
4 oz Chocolate
8 oz Victory

1 oz Bittering (Boil 40 min)
1 oz Flavoring (Boil 15 min)
1 oz Aroma (Boil final 5 min)

Safale S-04

Looks like 4.5lbs of pale 2-row for 3.3lb of the LME, but would that take too much away from the style of this beer ?
What type of grain should I use instead of the 2 row pale ?

http://beercalculus.hopville.com/recipe

Just subtract a few lbs. of extract and determine how much base grain you are capable of mashing. I would probably go with Maris Otter as my base grain or British 2-row. Halycon is a nice grain as well.

OK, a BB rep warned me that I may lose out on some roasted characteristics by doing this. I thought substituting grains for extract would help my flavoring rather than taint it. Are there more appropriate grains than pale 2-row, to use with this kit to replace the amber extract ?

Also, I include the specialty grains in with the mashing grains, correct ?
Therefore, I will use approx 8.25 quarts of water. (5.5 lbs x 1.5 qt/lb)