Couple of questions

Greeting all. Need experienced advice again. Looking at buying a 73 Duster bench seat w/tracks to put in a 71 Demon. Will it bolt right in? From auto to 4 speed shouldn't matter, right? Concerned about different floor platform from 71 to 73. Catalytic converter issue? Second question is looking at 69 340 intake manifold. Casting Numbers are correct but what else should I check on it besides any Stripped threads. How to check to see if it has been milled or altered? What to look for to check for obvious faults that would prevent it from functioning as it should. Thanks