Pull the dizzy out. (is it an MSD billet or stock HEI?) My car had an MSD Pro Billet
Verify cap is installed correctly (MSD can be installed backwards. There are round and square alignment pins)
Verify rotor is good and installed correctly (Same as cap above)
Verify the cam gear pin is not sheared or cam gear is worn due to dissimilar components. Example: Steel, Bronze, Fiber
My MSD Dizzy needed a new magnetic pickup due to moisture getting under the cap. It would work sometimes and then no spark etc. I rebuilt the entire distributor
Verify TDC on #1 piston (Pull valve cover)
Insert distributor into block taking notice of 2 detents to seat it. One is aligning the oil pump drive pin to cam gear. Notice where rotor is pointing
Remove dizzy
Turn oil pump drive pin to align with cam gear close to where rotor will point to #1 plug wire on cap (I mark the side of distributor) . ( you can use a long screw driver)
Reinsert dizzy into block with rotor pointing close to #1 plug wire on cap after the second detent. Advance the distributor about a 1/4 turn. (This sets it at about 8 degrees advanced to alleviate backfire through carb etc)
Tighten distributor nut to create friction but allow you to advance with pressure. (keeps it from returning to original position)
Hook up timing light
(Buy yourself a remote start button. They're cheap and make life easy)
attempt to start car and hopefully set timing.
Set timing per your cam card
If it works have beer.

Good luck