There have been some good suggestions as to where to buy parts. I'd like to ad that the engine is only 1 little part to the equation. You can build a 600 HP monster but if you stick it in a 74 Duster (or any similar vehicle) leaving all other parts of the drivetrain and suspension stock you won't outrun much. Figure in your budget the trans. and suspension/tire mods needed.
BTW: here is the run down on the cubic inches gained with a 4" crank
Stock bore 340 with a 4" crank = 410 cubes
.030 over bore " " " " = 416
.040 " " " " " " = 418
Then if you have the money you can go with a 4.125" crank to gain approx. another 12 cubic inches.