Points well taken. I've built stuff all my life and, while welding is fairly new to my other endeavors, I think that is doable also. I think that the way to go about this would be to start from the inside and work out so that you are not trying to shoehorn something in at the end. PTs strike me as fairly simple machines that are beefed up in all the critical areas. No frills, just get it done where it has to be done. Certainly making it better than factory bought would be primary to this project, but I somehow think that one could take the basic concept, shop smart for components, get advice from people like yourselves, and with a little ingenuity and professional advice you could make the ultimate (until someone improves it) articulated 425 sized tractor.
For instance, the tub size might have to be a little larger to accomodate a larger engine. But with that you might be able to solve some muffler and heat problems with just a tad bit more space. Wheel size and motor size is mostly vertical so you're not really adding much there. PTs aren't ZTRs so a little loss in turning radius or overall length of the machine isn't going to change a whole lot given that everything is out front anyway.
I am probably crazy to even consider this, but the simplicity of the PT design almost begs to have a whack at it. Power Trac makes what they make because they are the only act in town, and they have seemingly plenty of buyers to satisfy their corporate goals. Dropping a diesel in a 425 sized machine is logical, and quite frankly, doable. For Power Trac, it's a matter of "why bother" when we have a 1430 waiting for you.
It would be an intriguing project. Personally, I think it could be done for maybe less than the price of a 425 if you looked at it as a hobby project and didn't factor in a dollar amount for your time. One thing is for sure. I'll have to take a crash course on hydraulics!
Your thoughts and ideas for making this souped-up 425 would be welcome.