Something just isn't right.A 10:1 340 should be very happy at 12 or so initial timing.Somebody must have welded the slots in the distributor to reduce the mechanical to 10 so 25 initial gives you full advance of 35 degrees.I'm guessing it's a very lo compression engine to be happy at that.
If it is a true 10:1 340 and your figures are correct i would just buy a stock mopar distributor from Rock Auto for about 50 bucks and install it as is or change the springs for a bit quicker advance.