From what I've seen you have 2 options for running 6 throttle body inlet manifold.
Toyota used MAP sensing on the last of their 20 valve 4AGE engine and they used a vacuum point from the cylinder head side of each throttle body, feeding into a small reservoir to try and help dampen the pulsations coming from each cylinder. My personal guess is that the engine management system looked more at the throttle position sensor value than the MAP sensor value.
Typically speaking, you will have quite low resolution with this set up. It won't take much throttle movement at low engine speeds for the MAP sensor to show atmospheric pressure. The MAP sensor may also show quite a high voltage value at idle. So the difference between idle voltage and atmospheric pressure voltage may be quite narrow. Some computers can be configured to look at a very narrow voltage/sensor value range, but plenty do like to have as much of their 0 to 5 volt range to give acceptable mapping resolution.
The other approach that is much better in my mind (I also know that Moto Guzzi applied this method) is to base the load sensing on a linear throttle position sensor and use the MAP sensor to provide an atmospheric pressure input to the computer. That is, the MAP sensor doesn't try and measure manifold vacuum, just atmospheric pressure and any variation it may have with changing weather conditions and altitude.
To set up an idle speed control valve will require a small reservoir that is then plumbed to each of the post throttle body runners. The idle speed control valve would be attached to the reservoir. Use a proper, car manufacturer type of ISC valve rather than any old solenoid valve.
Basically the it's the same set up as the common feed MAP sensor set up.
2 things to note about this. Make the plumbing as short and as even as possible. And you will probably struggle with a very low MAP sensor signal if you use this idle speed control method and primary load sensing via MAP sensor.
My personal preference is for the Australian made Adaptronic computers because that's what I have some experience with. They are also awesome value for money, have multitudes of configurable auxiliary outputs and digital inputs to help the computer run the engine/car (little things like A/C, power steering via a pressure switch and electrical load (head lights) inputs) to help maintain perfect idle speed control, can be configured for nearly any weird a$$ factory crank angle sensor you can name, with out having to change sensor type or trigger wheel. And they also provide excellent support and firmware upgrades (for free). They also have a 164 engine base map for you to get started with (though that is based on the factory inlet manifold. The CAS settings alone will safe time and money)
Haltech also make some very well priced and very capable range of computers.