I just did this conversion on my 87 Comp. I went with the DUI HEI distributor from Performance Distributors. They build the distributors specifically for your application so as to have the proper timing curve and advance. What is your total timing above 3000 rpm's? You should see between 30-35 degrees. If you only get 15 degrees total advance from the distributor and then you base timing will need to be at least 15 degrees and that's a bit much according to most opinions. My DUI distributor was calibrated to have 24 degrees of total advance so when I set my base timing to 10 degrees like we all seem to do then I get 34 degrees total timing above 3500 rpms. Most all of the automotive distributors rely on the vacuum advance to get the total time to where it needs to be at the higher rpm's so I would be interested to see how much total advance you get with your base time at 10 degrees.
1987 Supra Comp TS6M
GT40p Heads, Performer RPM intake, DUI HEI Ignition
ACME 541
Big Air Ice Tower