if i were you idd rip the head, sump, dizzy and all covers off + Welsh plugs,
remove the cam, and crank, then blast it with a pressure washer,
buy yourself a 4" honing tool ($10-15 at supper cheep) and some wd40/crc
and give the bore a de-glase while your at it (while it out it worth it) use a cordless drill on slow speed and do figure 8's
clean up the stock pistons, but replace the rings,
replace the big ends and mains bearing with stock replacement, but leave the cam bearing(to hard to change)
replace the timing gears, water pump and oil pump, and fit all new gaskets
whilst the heads off send it to the reconditioners and get satellite inserts fitted to it and valves regrind (normal 10 per valve and 50 per insert)
http://www.acl.com.au/web/aclwebsite.nsf/8df7d3f7b605537a4a2569660082eda8/a0e0db0bd31ef0894a256967000e2235/$FILE/EZYFIT.pdf (pg 38)
http://www.aclperformance.com.au/Holden138.htmthen convert the dizzy to elect.
http://www.ebay.com.au/itm/Pertronix-Ignitor-Holden-Grey-motor-6-cyl-Bosch-/360401049411?pt=AU_Car_Parts_Accessories&hash=item53e9939743this will not be like a rebuilt motor but a freshen up, and will allow also make it more reliable and ulp friendly
head should be around $500
bearings/ rings $170
gasket set $150
timing gear,water/oil pump $250
dizzy $130
this is what i roughly paid last time i did one this way
if you wish go the next step you'll add another 8-1500 for machining, 2-400 for pistons + 300 for cam&bearing
depending how far/hard you wish to go