If I understand your question, you are looking to retain the specs of the original kit and are willing to adjust your volumes to target those numbers.
This being the case, I would start with your customized equipment profile and plug in the ingredients as per the kit. See where the target numbers come out. You can then decide if you want to adjust your volume to the fermenter to get your ending numbers to match the kit. Once you make this adjustment for the OG, if the IBU is anywhere close to the recipe target (+/- 5 IBU), I would leave it as is and brew. If the IBU is off a bit more than that, you may need to make an adjustment in addition times to get it within range.