What year is the block? with aluminum heads helping cut comp and the block could be your culprit. I chased a problem for close to a year with the e -58 engine I have today I Could not get the car to run a reasonable time I was convinced that timing has to be total of no more then 36-38 degree's In the end after talking to several older mechanics they gave the instructions give it all it will take then if it talks back drop it 2 degree's I have lived with that ever since currently running 45 degree's total and have run as much as 47 with racing gas. Car is not fast but a 7.5 motor running with a backyard rebuild with same pistons has run 14.3 at 96 in a 4300 lb car By the way not real issue to date with timing that high.

My 340 on the other hand does not like more then 34 degree's with 93 octane Use to be able to run near 40 with good old style Sunoco 260 1970

Conclusion do what works its your engine