Decurion

a holley jet #65 does NOT mean a .065 opening. Some aftermarket jets are measured this way, but the stock holley is not. You say it backfired when you were originally tried to start it, so it may have blown the power valve. The easiest way to check this is by turning the idle mixture screws in all the way. If it stalls before you get all the way in (no fuel flow) chances are good that the power valve is fine. If you can run both of them in all the way and it keeps on running fine, its blown. Also dont forget exactly how far you turned each screw so you can put it back to its original setting. Your choke should be wide open, straight up and down. To eliminate the possibility of a 'choking choke', wire it open somehow. Make sure your floats are set correct. To do this on a holley, get the engine warmed up and while its running, pull the sight plug off the float bowl. The fuel should just just dribble out of the hole. No fuel coming out means the float is too low, flooding out means its too high. If while setting the float you cant seem to get it low enough and have run out of thread, pull the needle and seat out and clean or replace. Once you have done all this, go back and reset the idle mixture screws using either a vacuum gauge or 1000 rpm tach to achieve the highest vacuum and/or highest idle rpm, then reset idle speed as necessary using the idle speed screw. Dont forget that for your carb settings to be accurate, you need to have clean plugs to start with. Fouled plugs make it basically impossible to tune your carb, especially at idle.

Also, you say your base timing is at 6*, thats pretty weak. Try disconnecting any vacuum source to the distributor and bumping it up to 10-12*, then reconnect vacuum source.

Fine tuning a carb can be an intimidating task, especially if you dont have a lot of experience with it, but stick with it and it can be VERY rewarding in terms of power, driveability, gas mileage, and overall fun and feel of the car. Lastly, unless you have a really big cam or odd combination, Holleys stock jet sizes are usually pretty close to what you will need.

Decurion

keep an eye on this thread too https://mustangforums.com/m_2387465/tm.htm