Like I always do, I'd strongly recommend buying a new stereo for your B350 from Crutchfield.com. When you buy from them, they'll tell you which stereo or speakers will fit so you don't waste time and money trying to install stuff that's either too big or is shaped differently than the places in your dash and doors. They have specific info for over 25,000 vehicles. I checked to see if they had information on what will fit in your 1989 B350, and they do. I'd bet that they'd also have the info about the amp and its wiring.
Over the years, I've put in four car stereos in different cars with their help. Other than being an extremely satisfied customer, I have no connection to them.
Purchases from Crutchfield include detailed, vehicle-specific instructions with photos that show you how to disassemble the parts of your particular vehicle's dashboard and then install the stereo. They also include a dash kit for your vehicle that includes any necessary dashboard trim pieces, brackets, and a wiring harness to make wiring it up a lot easier. Specific speaker installation instructions are also included when you buy speakers from them. These instructions are developed by Crutchfield's team—none of them are the generic crappy instructions that often come in the boxes.
Purchases from them come with lifetime US-based free tech support and if you run into a problem with the installation, help is just a phone call away. They stand behind the stuff they carry and they have a liberal return policy. Their website also has the inexpensive hard-to-find tools to pry off dashboard parts and door panels without damaging anything.
Yes, you can probably figure it all out yourself, but it would probably take a very long time, serious frustration, some bruised fingers and maybe some broken parts—all while you try to install stuff that won't fit, no matter what you do. It's just not worth it.