Aha, with my old Dodge van with the engine directly between the front seats, it was always very warm inside. I'm a bit surprised that with, such a small doghouse, it's getting so warm. Maybe that's just how it is, but I was wondering if maybe the insulation hasn't fallen off the inside of the doghouse after 23-years. Maybe good to talk to a dealer about this, or at least pull the doghouse out and take a look. Also, the seals around the edges maybe worn or not seated properly.
Where I live it's still in the low to mid-90s, so waiting for September to cool down so I can work more on my van too. I have one of those regular white GMC cargo vans, but it looks like the dimensions of yours are similar, eg 79" across. It's nice to have all those windows.
https://www.google.com/search?q=1995+Dodge+van+dimensions
What I would recommend is "initially" try not to build things too permanently. Rather, toss in things temporarilly, take a trip or two, and see how you like it. I made an initial arrangement in my van with the bed longitudinally front-to-back, and after 6-weeks on the road decided it was just too cramped inside. I've also watched many videos of van conversions, and many people spend a lot of time and many money on their conversions, and end up putting so many cabinets and whatnot inside that their floor space goes to nothing. Floor space disappears fast. I had 3'x3' before and 3'x6' now, which is room to put a folding chair, move around, &etc.
My major suggestion is to check whether you can put the mattress across the back rather than front-to-back. I found that with the std cargo lashing boards removed, it is 76" across just above the wheel wells in my van, so I could put a nice 8" foam mattress across. If I put in a lot of insulation and paneling like most people do, then it's only 68" across and too short for the bed. You can cut/shape these foam mattresses with a carving knife.
https://www.amazon.com/gp/product/B006L9QN4G