Don't put a big block in, way too heavy and all kinds of trouble to fit. Get some headers and a good carb and intake manifold, like Edelbrock. Dual exhaust, street performance type of camshaft. You can make that 318 plenty fast with just easy things like that.
Also, get some KYB shock absorbers and if your car doesn't have disk brakes in front, find a 73 or newer Dart that does and just swap parts.
If you want to lower the front, all you have to do is turn some bolts under the lower control arms, but not too much, and make sure the alignment is good. Go to a junk yard and get some 15 inch wheels cheap, even just steel 15 inch will be better than those 14 inchers.
Put polyurethane suspension bushings in. Energy Suspension or some other good brand, will make it handle much better and safer.
Get a subscription to Mopar Action magazine. The best tech advice you can find is in there.