Four tubes can be done. At least there is room, if you mount the four sockets to a new deck plate, to squeeze enough air across all four. One thing the 'hacks' usually seem to overlook is the need to blow air across the UNDERSIDES of the sockets. The stock fan does this. That's why the deck is cut out where the fan blades are. So some of that air blows across the UNDERSIDES of the sockets. One after another, we've seen a 'converted' SB-220 with fans that blow ONLY on the tubes above the deck. Strangely enough, the complaint is that the tubes "drop solder" out of the cathode pins. Kinda like this.
Or this:
The new HV power transformer will have to be twice the size of the old one, making an external cabinet necessary for the power supply. This leaves plenty of room for the larger filament transformer, once the old one is removed. Sticking to four tubes also permits you to save a few bucks using a ten-Volt center-tapped transformer to run the filaments. One pair of tubes goes on each side of the ten-volt winding's center tap. This permits you to use the stock cathode choke, since it's built to carry the 29 Amps of filament current for two tubes.
If you put five tubes in that cabinet, you'll end up starving one (or more) of them for air flow. What it really needs is a large (8-inch wheel) low-speed blower. A high-speed motor will be too loud. A quiet blower will have to have a large wheel to deliver the air flow.
And any custom work that gets quoted at a cheap price is not being done well. If you cut corners on this setup, the thing will fail, over and over. Gets expensive to own, even if it was cheaper to acquire.
For the same 3 thousand or more bucks it takes to do this right, you could have a Henry or Alpha that will run for years without breakdowns.
I'm with Jay on this proposition, but I'll go one better. If you need more than two tubes, what you REALLY need is BIGGER tubes. The only reason to use more than two would be if they were REALLY cheap, in the Watts-per-dollar department. If so, this would make up for the high cost of custom work. Don't know where you'll find 3-500Z tubes, or all the other parts you'll need cheap enough to justify what you'll end up with.
73