what is your opinion this combo?

It is possible - is the flat of the piston above the deck? If it is zero deck height .039" gasket thickness should be adequate. What rods do you have?
The other thing to look for is any rubbing on the pushrods. Magnum heads are set up for 1.6 stud mount rockers. I think you mean you have 1.5 stud mount rockers which will put the pushrod closer to the intake side of the pushrod clearance hole in the heads. Secondly, you are running a flat tappet cam which makes the angles even worse with the magnum heads = check pushrod clearances at closed and open lifts for rubbing/ interference.
What initial advance and total mech advance do you have? What rpm do you have total mech advance?