Definately buy that 65 bb727 if he only wants $200 for it. Make sure it has never been stored outside in the rain, pull the bottom pan off and inspect the inside for rust and/or any massive damage. Also try and get the kickdown linkage, very important and hard to find by itself. You can reuse your shifter cables from your car, the 65 cables won't work unless they are for a console and you convert your car to a console, which in a four door would be kind of cool.
Like Rapid said a 440 or 451 (400 motor with a 440 crank) would be the easiest to build and make good torque and horsepower. Start with a forged crank and pistons for years of dependable service, and a good foundation for later modifications. You can find good deals every once in a while here on moparts in the big block classified section or
http://www.racingjunk.com or greedbay. With $2000 to $4000 to spend, you can easily swap to a big block, just keep your eyes open for good deals.
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Correct me if I am wrong guys......an Ebody 8 3/4 will bolt in?
It would be kind of a tight fit, you would have to pull the leaf springs out an inch on each side (b-body spring perches are 44" apart and e-body are 46" apart).
If you want a drop in rear axle assembly for the 63-64 b-bodies (overall width is 60 7/8"). Then my first choice would be the 65-67 b-body rear axles since their width is 59 1/2" (you gain about 5/8th of an inch clearance on each side, this may not sound like much, but when you go to cram a fat tire in the rear wheel wells every 1/8th is needed, unless you are going to split the rear wheel tubs and move the springs in) and then my second choice would be the 68-70 b-bodies their width is 60 1/8" (almost the same as the 1964 width).
