OK, here's what you do.
Make sure you've got the motor around 10-12 degrees before top dead center on #1. Keep in mind that the zero mark on the balancer will line up with the timing mark twice....at TDC compression for #1 and at TDC compression for #6.
The easiest way to make sure you're at #1 and not #6 without removing a valve cover is to pull out the #1 plug and hold your thumb over the hole while you use a breaker bar to turn the motor over clockwise. You will only feel pressure if the piston is coming up on actual TDC for compression on #1.
Once you're sure you're on the compression stroke for #1 at zero on the timing tab, turn the motor back slightly counter clockwise to set the marker on the balancer to around 10-12 degrees BTDC.
Now use a flashlight and look down the distributor hole to see the end of the oil pump shaft. Get a long screwdriver and use a Sharpie to draw a line on the end of the handle parallel with the blade. Turn the oil pump shaft until the line on the end of your screwdriver is pointing at the #5 intake valve, or at least generally at the #5 cylinder (on #5 the exhaust valve is about inline with the header port, the intake valve toward the firewall from there).
Now, take your HEI and put the cap on it. Come straight down from the center of the #1 post on the cap. Use the Sharpie to draw a line on the distributor body where the cap meets the metal directly below the #1 post. Use a tape measure to draw another line to the right (counter clockwise when looking down) of that one 2-1/4" away on the distributor body. I'll call that mark #2.
Hold the distributor over the hole with the leading edge of the rotor (left side looking down) lined up with mark #2 and the vacuum can approximately pointed 45 degrees across the passenger side of the motor. Drop the distributor in the hole and once it is about about an inch from bottoming, the distributor gear should mesh with the cam gear and the rotor will turn clockwise to line up with the mark you made for the #1 post. You may have to wiggle the distributor and rotor to get it to drop down in the oil pump shaft and then it should bottom all the way out.
Remove the two screws holding the rotor on and remove the rotor. You should see two sets of triangles cast into the distributor body and the rotor assembly which point towards one another. Install your distributor clamp so that there is some resistance when you turn the distributor. Turn the distributor a little so that the points of the triangles line up with one another and then tighten down the distributor hold-down clamp. You may want to hold the rotor assembly to keep it from trying to "open up" while you turn the distributor and make sure it stays against the stops.
Once the rotor, cap, and plug wires are back on, the static timing at 10-12 degrees BTDC should be close enough to fire it right up.
A lot of this will make sense once you start working on it.
I have a '69 with HEI and it does not contact the firewall so hopefully yours shouldn't either. I've got the correct height trans mount (M20) and correct frame and engine mounts. If yours happens to make contact, you either have to raise the tranny, lower the motor, or dimple the firewall. If you're doing this without the tranny and tranny mount installed, then the motor will tend to lean back in the mounts so the distributor would contact the firewall.