Tricky ground there....
I will assume this is a Vin C 3800 that you are going to use.....
First the 1995 van used 9.0 : 1 compression as opposed to the 1990 3800 at 8.5 : 1 compression... The 95 3800 also used a roller pivot as opposed to the 90's pivot for the rocker arm...
The 95 used an internal PCV system where the 90 did not...
For all intent purposes, you could install the 95 heads on the 90 block and install all the 95 parts including the exhaust manifolds and the like.... I am quite sure that all of the bolt bosses are the same, but you would have to make sure of that...
Not so sure I would bother to go this route... Then you'd need to use the 90 balancer and flywheel.... The crank sensors are different... 93+ is non adjustable, 92 and under are adjustable...
In all honesty its way easier to find a 93-95 Vin L 3800 and drop it in... 95 was a very funky year as well for the engine and PCM....
I am sure it can be made to work, but even me being me this is one I would shy away from due to the level of work involved... I think it was easier for me to drop in my series III then to do the swap your thinking of..