Vali 68,
Ok Bud, here is what I did and this worked for me, I actually did the conversion and all works just fine.
1965 Valiant Signet, 225 /6 904 Automatic. I removed the /6 and Transmission, gave those away. Needed new transmount, cut notch in passenger side. Need a center link with the V8 drop. Motor Mounts from Shumachers.
Installed 1966 273 V8, installed 904 Auto Trans from donor 1973 Duster 318 Car.
Get externally balanced torque converter with LA flex plate.
Need a B&M Cable Shifter or a shifter conversion kit for the stock shifter, either one works fine, although you need to modify the stock shifter for this to work, but you will see nothing externally done. But it will work with a single cable. OR you can get a complete 66 Barracuda console shifter and keep the rod system, this works too, can get pricey.
Need a V8 accelerator set up, or you can fab with Lokar set up, same for kick down linkage.
Need to do the obvious with the electrical.
This is what I did to make a Slant 6 a V8 Car.
Is your car a V8 or /6?