CP4819 289/302/351W & early 351C Ford tubular engine crossmember (each) $59.00
2310 289/302/351W & early 351C Ford Motor mount pads - Rubber (each) 17.50
Level the truck. Bolt the engine and trans together as a unit and swing them into the engine bay with the tubular crossmember bolted to the motor. Place the motor in the engine bay on a 2-3 degree downbubble. If you are using a carb, bolt the intake manifold to the heads and use the carb mounting pad as level. With the carb pad level, the motor will be on a down-bubble to the rear like it is meant to be. When you have the motor leveled and down-bubbled correctly, tack weld the ends of the tubular engine crossmember to the frame of the Isuzu. You may have to cut out some or all of the original Isuzu engine crossmember to get the room you need. You can rebuild another crossmember in addition to the tubular engine crossmember if you want to, after the engine and trans are located. Use 3" x 3" square tubing, 0.120" wall thickness. You can also move the motor/transmission to the passenger side of the engine bay a little if you need room at the steering gear or whatever. If you do this, watch the trans and bellhousing in the driveshaft tunnel. You want to leave some clearance so that the components are not up against the sheet metal to generate noise. WARNING: THE ENGINE AND TRANSMISSION CENTERLINE MUST REMAIN PERPENDICULAR TO THE CENTERLINE OF THE REAR AXLE. THEY CANNOT BE LOLLY-GAGGED IN THE TRUCK.
For the transmission crossmember, you will need an Isuzu crossmember and a Ford crossmember that will accept the 3-speed rubber transmission mount. Cut out the middle of the Ford crossmember. You want just enough of it to bolt to the rubber mount and extend out a little on each side. Whatever length you end up with for the middle piece, you want to cut that much out of the Isuzu crossmember. You will not have the center piece from the Ford and the two side pieces from the Isuzu. Bolt each side piece of the Isuzu crossmember to the Isuzu frame assembly where they normally bolt in stock and bolt the middle piece to the tailshaft of the trans with the rubber mount. Once you get the motor/trans in place, just connect the 3-piece crossmember together with whatever material it will take to do it and weld the whole mess together. Presto, custom trans crossmember that unbolts like stock.
I'm just skimming across the top of an engine swap for you to give you an idea of how to go about it. There will be many details for you to figure out yourself.