The way to give your existing engine greater ability is to go in the opposite direction to more ground speed.
Putting on the 400 motor means a larger internal displacement, slower turning motor that will deliver higher torque to the wheels. If that doesn't do it for you, then go with a smaller internal displacement pump. You will go slower, yet again but the engine will not bog down as badly.
So let's look at it this way...
Suppose for a second that you have a .62 cubic inch AA pump and it has a theoretical GPM of (.62 X 3600 = 2232 divided by 231 = 9.66 GPM @ 0 PSI)
If you want to slow the tractor's speed down by 30 percent, then you need a pump that puts out (9.66 X 70% = 6.76 GPM)
A pump in the vicinity of .42 cubic inches would give you (.42 X 3600 RPM =1512 divided by 231 = 6.55 GPM @ 0 PSI)
As an example, this pump has the sort of specs you would be looking for.
http://www.grainger.com/Grainger/HALDEX ... Pid=search
It will put out as much as 4000 PSI for you. I can't tell you what the drive motor will handle or even the trans-axle gears but those are big gears and the axles are pretty robust too. Pulling is always about pushing what you have until something breaks and then you make that weak link stronger and push again in the hopes that you can win without breaking something. You could check with Surplus Center's site to see if they have a pump like that for less money.