Slant six dies in gear

Will post some pics a little later. First noticed a starting issue about 4 months back right before my coil died. Replaced that and still had ignition issues. After replacing all resistors, wires, ignitions switch etc it started first try. Then came an overheating issue. I live out of town and after about 20 minutes of highway speeds 60 mph/ 100km. The coolant would bubble and overflow into the overflow reservoir. I have a rad cap that has a temp gauge on it and that as well as a temp gun read 290 degrees. Once that was discovered I started replacing things. I started with the cheapest things, (college budget) thermostat, then water pump, rad fan, rad itself. It’s also worth noting that the car would overheat even at an idle so I looked into the distributor. I ordered one and discovered it had a crooked shaft (rebuilt part) as I’m out of town and takes a long time for me to get parts I rebuilt it myself using good parts from my old distributor. It’s seemed to fix majority of the issue but the car was still overheating. After getting desperate I decided to pull the head. This is where I discovered a leak between cylinders 1 and 2. Made the choice to replace the head instead of rebore it. It was pretty far gone. As that was out I figure I’d replace the timing chain as well. Once those were in I tried starting it and no dice. After a couple weeks of no start and me messing with valves and timing I got it going with no overheating issues. Thank god lol. Although this is when it was discovered that the idle likes to die when you go from P or N to gear. Even 1st or 2nd. As I am a college student and it is my daily I have still currently driving it. So long as you hold the gas slightly to keep gas flowing it won’t turn off at lights and such. The worst of it is trying to get it going after a night of being parked. Sometimes take 6-7 tries. Even if I’m slow on the gas or even have my foot on the gas and shift from N to D at the same time.

So the problem started after the head and timing chain was replaced. Sounds like a combination of carb (and choke) and ignition. Do you have electronic ignition? Does your distributor have vacuum advance? Have you tried to rebuild the carb? Check choke to see that it is closed when cold and open when at opperating temperature. Assuming a 225, I'd start from the 1975 tune up guide, set timing to TDC and idle to 750 in neutral at opperating temperature. Plug all vacuum taps at the carb. This is an iterative process.
1. Adjust idle screws for highest vacuum and rpm.
2. Check timing with vacuum advance plugged at the carb.
3. Set timing.
Then back to step 1 and repeat till it all lines up.
Set fast idle to spec on the highest point on the cam for when the choke is closed.