1978 f150 with the 4.9L inline 6 with an automatic C6

I recently completed an overhaul on the engine and put it back in the truck with an Offenhauser c series intake and 2 barrel Holley 2300. I completed the break in period and attempted to put it into gear where I shifted down into reverse causing the engine to just about die. I gave it a little gas and it stayed alive long enough for me to put it in drive. When I did that, foot on the break, the truck started burning rubber in my drive way. I killed the engine and since replaced the shabby custom throttle linkage I had made the other day. I put some return springs on it and the pedal is responding much better. When I tried to move the truck again today, I clicked down into reverse and it died just about right away.
I read that it could be due to a low idle setting, but the idle actually seems to be relatively high just based off of what I can hear. I don't have an rpm gauge and when I've adjusted the idle mixture there doesn't seem to exist a sweet spot between struggling to stay alive and idling too high. I'm not accustomed to what its supposed to sound like though, so I'm not sure. The throttle kickdown linkage was also never hooked back up, as I read it's not really necessary. It may also be slightly off on the ignition timing, I have not used a timing gun yet.
With the truck behaving the same way in reverse, I'm concerned it will do the same in drive and I'd rather not put it to the test and lose control. Any thoughts as to what is happening? And a side note - after the engine runs for about 60 seconds a considerably loud whine persists, it sounds like a flute playing a very high note.

 

Sounds like you have a bad vacuum leak. check the obvious stuff first hopefully it's not an intake leak.

the timing is very important. if you don't have a light for now get it running and move the distributor until it runs the best. then retard it back 5-10 degrees.

 

What makes you think it's vacuum? If you don't mind me asking

 

A vacuum leak will produce erratic engine operation and a sound like you described.

If the leak isn't horrible usually if you get it idling you can slowly cover carb with a rag and if you have a leak it will speed up, if you don't it will die.