Corporate Finance / Project Analysis Case Study?

hey guys - so as part of an infrastructure IBD interview, I have to complete a project finance case study. it's pretty complicated and im not 100% sure how to approach it. just looking for some opinions.

the case study is basically asking to figure out how to finance the construction of a bridge, which is operational for 20 years.

the bridge has a construction cost of $800 million, spread out over 3 years from 2015 to 2018; operational costs of $7 million growing at a rate of 1.9%; a one time reward of $500 million paid upon completion; and revenues of $9 million growing at an annual rate of 2.5%. which pay off the operating costs and pay dividends.

I can finance it through a bank loan at 1.50% and a credit spread of 70 bps per annum, which prevents me from taking dividends. it also charges a commitment fee, which is an interest rate of 35% of the credit spread of the bank facility payable monthly on all undrawn amounts.

I can also finance it through bonds, with an upfront free of 200 bps, which pay 2.4% per annum and have a credit spread of 200 bps. During the construction period, they pay interest only. They must be fully amortized by the end of the operations.

I also have $50 million in equity that must be invested in the project.

how do I find the optimal amount of bank loan/optimal amount of bonds to issue to fund the project? I'm trying to build an excel model to calculate this and to maximize IRR. not looking for someone to solve it, just a general idea of how to approach it or some resources I could use. any ideas?