The HEI is in the same vain as Ford's Thick-Film-Ignition system. It's old technology and not very reliable. In my opinion it is not the RF that is causing your problem.
As for the tach, a split bead of two in the right place should cure it.
One of the most often asked questions in the mobile forum goes something like this; Will I cook my (insert what ever electronic automobile device you can think of here)? The fact is, most automotive electronics are robust and well protected from stray RF, especially air bag controls. Notice I said most. The device(s) most problematic is car stereos and power amplifiers. No matter what name appears on them, they are basically junk when it comes to RFI suppression. In fact, a lot of the high end stuff is worse than factory stock.