last minute hop substitution advice

I'm up late brewing a summer ale based on Mosher's summer ale in Radical Brewing:
4 lb Marris Otter
2 lb Pilsener malt
1 lb 6-row (had some to use up)
10 oz malted wheat
0.5 lb turbinado
White Labs WLP023 Burton Ale Yeast

Mashed at 148°F for 90 minutes.

Mosher calls for Challenger additions at 60, 30, and 5 minutes and some Saaz at 5 minutes. I thought I had Challenger in inventory, but I don't. I'm going to switch to Amarillo. In this case, should I change the Saaz to Cascade? Or would Amarillo and Saaz go well together?

I'm about to start the boil so any advice would be appreciated!

I would do Cascade and Amarillo. The Saaz might be overpowered by the citrus of either Cascade or Amarillo.

Whatever is higher in AA%, Cascade or Amarillo, use that one for the 60, then use the other or a combo of both for the other additions. How many IBU's is the recipe shooting for?

It assumes the Challenger additions are 7.5% AA and the Saaz 3.0% AA. For a total of 43 IBUs.

My Amarillo is 9.6% and my Cascade is 6.0%. Sounds like I'll use Amarillo for bittering and Challenger for aroma.

You guys are awesome. I knew you wouldn't let me down!

You're going to go about 0.75 oz. on the Amarillo and then about 0.5 oz additions on the rest