Ok all this marking stuff is going to be invalidated once you change cams. There's no gaurantee that the timing gears on the new cam are going to be in exactly the same relation to the cam key as they were with the old cam. This puts the rotor in a slightly different position with the new cam, and therefore would put the housing in a different position in relation to the block.. so a housing to block mark would be misleading.
About the only meaningful mark you could make, is get the crank at 0 on the compression stroke, then before pulling the distributor, mark the dist housing exactly where the rotor is pointing. Then when you get the new cam in, get the crank back on 0 on the compression stroke (whichever method already describe above), and then you know exactly where your housing needs to be in relation to the rotor. after it's all done you should still dust off the ole timing light.
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'78 Macho T/A DKM#95, 460cid, SRP pistons, KRE 310 D ports,
3" pypes, Hooker 1 3/4" headers, hydraulic roller,
10" Continental, 3.42 gears
11.5 @117.5mph 3900lbs 
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