I start my injection at about 8 psi, but I've seen some folks use turn on points down near 5 psi. To some extent it depends on the built in delays in your injection system (how long it takes to start spraying a meaningful amount) and how fast your car builds boost.
Hmmmmm -- Not that familiar with the car (aware of it but never worked on one) -- Does it have standard distributor ignition (sounds like it) ?
Have you checked your distributor cap for cracks cross firing, bad ignition wires etc?
You might try pulling back the base ignition timing just a degree or so more if you can.
On the older iron with distributors you'd get that sort of behavior if the timing advance came in a bit too quick. If it is a conventional distributor, you may be able to find a shop that can put stiffer centrifugal advance springs in there (or lighten the swing weights a bit) to slow down the centrifugal advance a bit so it does not reach full advance as quickly.
Most cars of that and earlier periods had a base ignition timing some where around 10 - 12 degrees advance and then it would rapidly add centrifugal advance as the engine rev'd and it would have all the ignition advance in by about 3500 rpm.
It is much easier to get a car to detonate at low rpm than it is at higher rpm (combustion time is much longer at low rpm). So --- if the centrifugal advance came in too quickly, there would be a place in the advance curve near 3000 - 3500 rpm that you are on the ragged edge of detonation. On a turbocharged car the same effect comes up if you come on to boost too quickly.
Given what I know about your car, those are the things I'd look at:
Pull back the base ignition timing just a couple degrees.
Slow the centrifugal advance by using stiffer springs or lighter swing weights.
Widen your spark plug gap a bit to get a faster burn at low rpm, (check ignition wires etc too).
Change your turbo boost setup so it reaches full boost just a tad slower.
Good Luck.
I'll look around and see if I can find any info on your car.
If anyone knows the in's and out's of those engines it would be the guys down at Gary Donovans Dodge Garage.
http://www.thedodgegarage.com/
http://www.thedodgegarage.com/turbo_shortblock.html
You may want to touch base with them -- by the way read his pages on the plymouth reliant called "Relentless" very fun read. (second click down on the "my rides" in the left side of the page.)
Looking through my motor manual it looks like the distributor up through 1983 at least had the centrifugal advance setup. On the '86 it was electronic distributor ignition, but there is no example of the internals of the distributor so not sure if the above will be any help.
If you've never seen the inside of a conventional construction distributor here's an exploded drawing of a typical one, so you know what I am talking about.
This may not be useful to you, but some of the other folks may find it useful, as the pool of mechanics familiar with the older equipment is rapidly declining.
http://www.gordon-glasgow.org/Images/18.gif
#28 are the swing weights
# 29~32 are the springs that control how fast the swing weights advance the ignition timing at higher rpm's
Larry