Same here...
The 86 305 motor I put in my truck has no timing marks either(no pointer anyway--it does have a line on the harmonic balancer)--I think someone might have lost the pointer when they had the timing cover off,but a friend who works at a GM dealer said many trucks and vans didnt come with a pointer(my motor was from a G10 van),I set mine by ear and with a vacuum gauge,I must have it pretty close,it doesnt ping,even without an EGR valve,and it has good power and decent mileage..
.I just advanced the timing until I reached the maximum vacuum at idle on the gauge,and retarded it until it dropped one inch on the gauge--after a test ride,I decided to retard it a little more,as it was pinging a little,probably because I blocked the EGR valve off so my edelbrock carb would fit,and it seems to run good where it is--I would like a way to see if its really timed right though...
Another mystery on this motor is the fact I can see no place to put an oxogen sensor!!--no holes in the exhaust manifolds,no "bung" in the y-pipe,NOTHING!!--How the hell did the computer the van had with a feedback Q-jet get any info to adjust the fuel air mixture??--the van also had a multi-prong module HEI with no vacuum advance--I put my edelbrock carb on it and an early HEI with a vacuum can on it,and "de-smogged" it,so its not a big deal,but its got me baffled--I thought all EFI and feedback carbs needed an O2 sensor??..