Slant sixes can make plenty of power in even nearly stock form. Don't listen to the people that tell you different.
Pre-smog and post-smog (whatever that means in your neck of the woods) mean absolutely nothing. Post 72, slant sixes were mechanically identical (save for the spark plugs) until 1981 when the lifters were changed to hydraulic (finally).
Everything from one motor will bolt to the other. The only thing to watch out for is if you are going to work with a pre-68 motor. The opening for the torque converter/input shaft on the back of the crank changed in 68. Post 68 the only changes of note are the switch to hardened valve seats in 72, the upgrade of the mechanical lifter camshaft to a better profile in 71, the switch to hydraulic lifter valvetrain and cam in 81, and the addition of a port in the head for A.I.R. injection in the late 70s.
$500 is pretty steep for a used slant, but you have to do what you have to do. I got the last slant six I picked up for free. A couple guys responded to the ad I posted for you on ss.org:
http://www.slantsix.org/forum/viewtopic.php?t=49500Because you have a van, I would try and find a post-1981 slant six since it will have hydraulic lifters that don't require manual adjusting.
While you are in the middle of pulling the engine, do several things:
(1) check for timing chain stretch. Replace as needed
(2) verify that the cam is degreed properly, especially if you replace the timing components
(3) check if timing mark still accurately indicates TDC. The outer ring on the vibration dampener can slip leading to incorrect timing marks.
(4) If you can afford it, install 2 1/4 inch exhaust line all the way back
(5) run 5W-30 oil and a Wix 51515 (or Purolator equivalent) oil filter
(6) pull the pan and verify that the oil pump pickup is clean and installed with the correct clearance to the pan
(7) verify that the thermostat is installed, is the correct temperature, and is working properly
Preferred slant six tune-up parts and techniques are discussed
HERE, although if you get a 75-up head (AKA "peanut plug" head) you will need to use NGK UR4 spark plugs. Also, as I said above, if you get a post-81 motor, you will not need to adjust the valve lash since the lifters are hydraulic.