In your position, I'd just brew it, and see how you like the outcome. Many factors affect perception of bitterness - it's far from an exact science.
For instance, if the recipe has been stored for any length of time at room temperature, the hop alpha acids in some hop varieties will have deteriorated up to 50% in 6 months.
But let's look at the theory all the same.
You said you want to target 6 gallons, but the recipe calls for a target of 5 gallons, so let's use that so we're not complicating things by introducing dilution into the picture. The partial boil was 2.5 gallons, and full boil is 5 gallons. The OG given is 1.043, so 1.040 in round figures.
John palmer gives a table showing the utilization factor for hops for different boil times and gravities. The recipe has two additions - one at 60 mins and the other at 45mins. The reduction in hops required going from a partial boil to full boil is:
- for 60 mins: 0.176/0.252 = 69.84%
- for 45 mins: 0.162/0.232 = 69.82%
So, reducing both additions to 70% of their original, 1oz down to 0.7oz will, at least on paper, get you down to the same region of IBUs.
If you then wanted to scale this up to 6 gallons, then multiply the hops and grist in the recipe by 1.2.