I previously made 3 conversions from 6 cyl to V-8's. One was a 55 3600, the other my 56 3100, and a 57 Cameo. None of them had the holes in the firewall or were dimpled for the holes. Currently working on a 57 3100, it has the holes in place but only one of the holes in the aftermarket bracket lines up with the holes in the firewall. The owner located an original V-8 linkage and it fits perfectly.
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