The wiring kits that I have seen don't include a resistor wire for the coil but many have undersized wire if an HEI is used. I believe that GM recommends a #10 wire for an HEI, most of the wiring kits provide a #16 wire. That may contribute to your problem but I don't believe it would cause a 3 volt drop.
Fred

1956 3100 Pickup/Red/350/3sp OD/PS/Disc Brakes
1957 Bel Air Sport Coupe/Red/355/TH350/PS/Disc Brakes
2017 Silverado LT Single Cab SB/Black/5.3/6 Speed Trans
1947 Willys CJ2A w/F-Head engine