For sure you should remove the front bumper, easy but best to have a helper to balance it. The fillers come off easy afterward. If they are original fillers, they are going to be junk at this point and will likely crumble. Time and ozone is the enemy. Goodmark are the most OEM-like...
The block is different. 14 bolt pan and front cover on 84/85 hot air, 20 bolt pan and front cover on 86/87 intercooled. Also block is not tapped for turbo drain on 84/85. You can use the hot air long block and all but you will have to tap it if you replace the intake/headers/downpipe and...
For as much as you are going to spend, don't do it. Build an intercooled engine instead. Upgrading a hot air is not a great use of funds unless it is super important to you.
Yes it is safe. It just won't go into lockup. The RPMs won't drop in 4th. It will just be without overdrive. I wouldn't leave it that way.
If that ends up being the issue, use a 700-4R long plunger lockup solenoid. Best wishes.
I almost wonder if the torque converter solenoid isn't releasing after going into lockup. That usually happens around 55mph. It feels like sputtering until it releases. You can unplug it from the drivers side of the transmission and it won't lock up while unplugged.