make sure the length is right too!
ive also got a td cortina, which was when i bought it and still is all engineeded for a 351 clevo. i was going to put a 4.0l in it so i could drive it on my p's, but found the length way out. the rocker cover was the same length as the firewall to the radiator. and the 6cyl cortys allready had the firewall pushed back to fit the 250.
it can be done. but a hot 6 or v8 conversion could be easier.
1976 351c td cortina