OK well to get it correct, finding true TDC first following the link cobalt provided is essential. The timing marks most likely only go up to about 18 Deg BTDC so unless you have a dial back timing light you should make a timing tape as well, follow the other link directions.
Since your engine is stock you can try about 12 deg BTDC initial timing, yes then add another 22 with mechanical advance for about 34 total all in by 3000 is a good place to start for a heavy truck. These two setting are made with the vacuum advance disconnected. Then add in another 14 with vacuum advance for a total of about 48.
First off after you are confident that the timing marks are OK check and see what you have now as far as total mechanical advance and total vacuum advance. Then you can determine if you need to tweak the distributor. It is best to find out these values first then you can go from there. Post some results after you check em out.
It is a good idea to pull the distributor and check it out for wear and clean it up. If you find it to be worn out then that skip white 6500 is an excellent value, I would not bother trying to rebuild the OEM when you can get an new one for less than 50 bucks, thats just me. Some more links:
Generally speaking with your stock 454 you should not need allot of modifications to your distributor, I am not sure that an adjustable can is really necessary in your case but it certainly won't hurt to have one in there and doing so will give you a chance to inspect the other working functions of the HEI as well as ensure your timing is setup accurately and to your liking Take some timing readings and post them if you are so inclined, good luck.