I agree with Bob. To get a little more detailedthen this...
If your Valiant came with a slant 6 ( 225 ci) then you will have to change the "K" member in the front end. Go with one that is a factory V8 car. Don't forget to get the disk brakes as well. You can keep the factory shocks for the lowered look on the front end. Then get a hold of Schumacher Automotive Services for transmission and engine mounts for this swap. Next make sure you get ALL the electronics and sensors for the computers to work installed in the Valiant. Then start looking for a exhaust shop to fabricate a header to exhaust tip exhaust system. Valiants were statndard 225 slant 6 cars. So no real "basic" V8 exhaust system out there that I know of. Maybe some one else in here does. You will then need to start looking at the rear end. The new V8 will punish the weak 6 cyclinder rear end.
Looks like a lot of work, but not as much as some I have seen concerning conversions. Good Luckand keep us posted.
DCON426