How can I modify a fuselage to increase the payload?
You really only have two options; stretch or swell.
The Q400 already has a &quot;combi&quot; configuration option with 50 seats forward and a cargo hold taking up the aft third of the cabin, so if you want to just add another couple of plugs, one (smaller plug) forward of the wings and one aft, to get 74 seats plus a large cargo area, you can do that and just adjust the structural requirements to deal with the even longer flying pencil that results.  You can have a cargo hold aft and pax cabin forward with the appropriate weight and balance requirements and limitations for loading pax vs cargo same as any other split combi layout.
In the case of the Q400 itself, its configuration is already at the extreme limit of what you can do with fuselage stretches on that sort of 4-across tube diameter, having started with 36 seats in the original.  The plane suffers from tail clearance problems because of this, due to limits on how much the gear legs could be extended to account for the long tail (landing with less than full flaps - there are a couple of intermediate landing flap settings - has a high risk of tail strikes and pitch attitude must be very carefully managed in the landing flare).
I think you'd get a better aircraft by going with &quot;swell&quot; and increasing the tube diameter enough to get a 5 across cabin (around 2 feet), with 2 on the left side and 3 on the right. A 74 pax 5-across cabin would only need 15 rows, leaving the rest free for cargo, and this extra width plus shortened passenger area might even allow you to shorten the fuselage a bit, leaving some stretch-ability potential in your initial design (you might even get bonus points for taking this into account).