The oil pan might be a stumper. You'll have to research the web and see if there is a front sump pan. If not, you'll have to cut yours off about an inch down from the edge then reverse and weld back together. The pickup tube shouldn't be a big problem, most are pressed in and come up from the bottom. If there is a tab on it that bolts on you may have to cut that off and weld at a different angle, or leave a stub and drill it for another piece that you can bolt to the pump and stub at whatever angle you need. Depends on where the tube mounts on the pump and where the pump mounts. I'm a rarity -- not intimately familiar with SMCs (which the 4.3L is based on) -- I'm an AMC/Rambler/Jeep guy! I've done quite a few odd swaps though.
The front clip isn't terribly difficult, depends on how things line up and what you want. It will be a LOT lower than the straight axle. If you want the truck high you can sit the truck frame on top of the front clip from the van. That can be made to look good, but will require a bit of cutting and welding, and some steering shaft work. Much easier to keep the straight axle even if you have to cut and weld the oil pan. the oil pan is easy enough to take to someone if necessary and shouldn't cost too much to do.
That 4.3L will weight LESS than the old L-head six. The newer auto trans will weigh more than the old stick tranny. All in all I'd say you will be close to the same on drivetrain weight, maybe a bit lighter, but not likely to be heavier.