Questions regarding idle on early Y-block

[zuburg]

I'm a new owner of a 1956 T-bird, unfortunately, it has a 235 Y-block with a bunch of Thunderbird bits bolted on instead of a 292. It has a Holley 4v carb. The current issue is a high idle of almost 1,000 rpms. I figured out how to lower it down to about 600 and it sounded pretty good at that speed. The issue is that when I put the automatic in gear (either drive or reverse) it dies at that speed. I had to increase the idle back up to 900 or more to keep it from dying.

I read that with an automatic, you may need to have someone help you by putting their foot on the brake and putting it in drive while you set the idle. I assume if I did that, once I put it back in neutral or park, the idle will be back up to 900?

I do plan to do a partial transmission rebuild to fix some serious leaking issues. Would this possibly help the situation?

Where should I start to address this issue?

thanks,