Just a quick question, I just bought a 71 ltd for the motor to swap into my 79 f-150 in place of the blown 400. My boss told me yesterday that those old ltd's also came with a 390, and I was wondering how you tell the difference between the two. Up until now all my hot rodding has been on chevy or dodge cars and trucks so any blue oval advice will be a great help, thanks.  

71 should be a 400. If it is it will bolt in. If it is a 390 it will not bolt in. I would not use a 71 motor though because it was not until 73 that they started putting hardened valves and seats in them for unleaded gas. If you work it hard you will valve issues it 10 to 20K miles. Also that engine is not going to like 87 octane at all working hard and you will have to retard spark a lot if you want to use 87 octane in it and the result is going to be less MPG and power too.  

Might be possible to put the heads off your blown engine on the '71 if its a 400?  Solves the need for hardened seats and will also drop compression to more user friendly numbers  

Thanks for the info guys, I'll be doing the head swap, cause I push my rigs pretty hard, and 87 octane is the best you can get round here. Any more advice will be put to good use.  

Head swap is a good idea but I am not sure if Ford lowered CR with head or dished pistons (you will know when you get heads off) If it was with dished pistons, head swap is not going to lower CR much if any.