Stability of a center toon to an old pontoon boat

Here is the 1986 Godfrey Aqua Patio with some of the new seats installed. As you look at the pictures, you can see that the boat is bow heavy. The Evinrude 48 SPL is what I call, "a little putt putt motor". It needs replaced with at least a 90HP. There is just too much boat for it to do anything other than push this thing along at a liesurely pace. In 1986, that is what pontoons were built to do.

I put a couple of passengers up front and I found out the hard way that the buoyacy on these old 19" toons is not real spectacular. I had more water over the deck than I really expected. I was okay with it but it made the passengers uncomfortable. 

My solution was to add a third toon, but here is what I was told by an expert...

He said the 19" toons do not have nearly the buoyancy of a modern toon and the boat would become uncomfortable to ride on with a larger center toon. He reccomended buying another 19" toon and said that would definitely improve the performance and would not make the deck "unstable".

I find that everyone has an opinion. Is he correct? I just can't imagine a 21" or a 23" toon in the center causing this boat to be "unstable". 

So, what is your experience?