Lap belts are fine with child/baby seats, at least all the ones that I have owned.

Think about how you mount a child seat (with integrated belts) with a 3 pt belt. You have you use that clip to tie the shoulder belt to the lap belt. Essentially you are anchoring as a lap belt would. Yes, you are distributing the load over 3 anchor points then 2, but for someone under 50 lbs, that is not going to make a difference.

Only thing you are missing is a top tether, but many cars older then Model Year 2000 would not have these anyway. At least that was when Chrysler added that safety feature.

I would just make sure the belts are in good condition and not dry rotted or cut at all. Also, adjust everything so there is no slack. This is the tricky part with the old adjusters. It may take a few minutes to get it all dialed in. I cover my seats with an old folded towel, so the plastic child seat does not dig into the vinyl. They can have some sharp edges.

And all my mopars, with the exception of my a-body vert had 3 belts in the rear.