The 318 and 360 are similar engines on the same basic design. They do not interchange without some modifications. Physically where one came out, the other will go in. However, one is internally balanced and one is externally balanced.
Before I did anything else, I'd do a good tuneup with new plugs, wires and also a new cap and rotor. Get one with either brass or copper electrodes. This gives a touch more fire over time as the aluminum electrodes tend to corrode.
Don't worry about kicking down into second gear. That's your passing gear. You only have three gears. Find a vacuum gauge. Hook that puppy up and watch it. If it drops fast but you haven't changed your throttle pressure, you may have an exhaust issue. I tracked one down at my shop that even the dealer couldn't find. If your catalytic converter or muffler is partly rusted inside, a flap of metal can flip under heavy pressure and partly block it. When the pressure drops, it can flip back. Rust or a rodent making a home in your exhaust can cause issues under load.
As for a big block, it will physically fit but you'll have to change just about everything and maintenance will be a nightmare. Remember, this is based on a van, not a pickup. A little more room will be available if you use a 383 or 400. Those are low deck engines. The 413, 426 and 440 are raised deck engines.
Your best bang for the buck will be a gear change. If you have 3.55 gears for instance, 3.73 will wake the van up and your top end won't tank. 3.91 gears will eat fuel and also cut your top speed a bit.
Keep in mind, you have a brick on wheels. I had a GMC unit about like yours. It had 4 speeds rather than 3 but still would kick donw on steep grades.
