My plan was to weld some cross members across the frame to securer the motor and fab a coupler to attach it to the existing manual tranny
That helps. So you're looking for a motor to run at similar speeds to the gas engine, because it has the same gearing available - it doesn't need enough torque to move the truck with just the rear axle gear reduction. It can be bulky, because it has the stock engine space available.
Can we assume that the battery will be split in two packs, under the box floor and ahead of the axle, one on each side of the propeller shaft... where the fuel tank and muffler were originally? If that - or perhaps that plus a pack replacing the spare tire - is enough capacity, only the motor, controller (inverter if AC), and accessories need to fit under the hood.
Or if necessary I can have an adaptor plate made. Depending on what the motor needs to be best supported.
Conventional transmissions are rigidly mounted to the engine, and supported by the frame only near the tail end. Unless you're planning some unusual setup with a jointed shaft between the motor and transmission, plus a front transmission mount and two (front and back) motor mounts, you need to rigidly mount the motor to the transmission, so you need that adapter plate. They're available ready-made, to emulate many engines including those used in the S-10, and for many common motors:
CanEV adapters. I assume there are other suppliers as well, and of course you can make one or have one made.
I would suggest avoiding welding on the frame if you can. The stock engine mounts will likely be around the front of a compact motor, and somewhere within the length of a big "forklift" motor (such as the 9" diameter DC motors which madderscience mentioned), so a custom motor mount could be built to drop into the stock mounting points. I would take a tape measure to the truck (front face of transmission to engine mounts) and compare to the motor length (dimensions from one of the motor sellers).
Aside from supporting the front of the transmission, and the motor, you need mounts to take the torque of the transmission output. That's normally the job of the engine mounts.
Without a specific example, my guess is that this is one of the most common EV conversions in existence, with one motor in place of the engine in a compact pickup truck; even specifically the S-10 has been done many times. The only differences from one done half a century ago and today would be the sophistication of the controller and the type of cells in the battery. It might be worth looking for other conversions and copying selected features from them.
It would probably also make sense to look at the
S10 conversion kit from Canadian Electric Vehicles Ltd. (about halfway down the page), as a possible solution, as a model for how it could be done, or at least as a checklist of components needed. Of course they don't show how everything fits in the truck; the "how to" is part of what they are selling.