No such thing as an '87 GMT400. If you're calling it an '87 because it was built in '87, then what you have is an '88 - check the 10th digit of the VIN for accurate year model. J=88, K=89, L=90..
You can certainly paint that dash to go in your '93, but what you'll find is 88-91, the area behind the gauge cluster where the cluster connector goes is different. There's a few ways you can hack around this but that's the one big difference in those dashboards. You already have that one so it's probably best to proceed with it, but if you didn't have one at all, my suggestion would be look for 92-94 instead as they are the same as your 93.
SEM makes good quality paints for interior parts, go to their site and do some reading. They have good instructions on cleaning the parts ahead of time to remove any dirt but most importantly any grease/oils/etc. that would prevent good adhesion. Read, re-read, and follow the directions to the letter.
Richard