Mechanical fuel pump on an 87 block? My truck didn't have one. But I'm not the expert.
most of the blocks are still machined for the fuel pump and came with a blockoff plate on the TBI and Vortec applications, but there are some blocks that have the mounting boss cast but not machined.
as for what to look for- look for the one piece rear main seal retainer- it is an aluminum piece that holds the rear seal and bolts to the block around the back of the crank. the older two piece seal blocks don't have this piece.
also look in the valley under the intake manifold to see if it is setup for a factory roller cam. there will be 4 (i think) stands cast into the center of the valley that are tapped down from the top to bolt down the "spider"
that holds the lifter retainers down. also the tops of thel ifter bores are machined down flat so the lifter retainers can sit down flat.
under the timing cover, the area around the front of the cam will be machined flat with 2 bolt holes tapped for the camshaft retianer on the roller cam blocks. the non roller blocks won't have this area machined.
some truck blocks and 86 passenger car blocks with the one piece rear seal don't have the provisions for the roller cam, and others have the stands cast in but not machined. the only sure fire way to tell a one piece rear main roller block from a non roller block is to actually look- you can pop off a valve cover and shine a flashlight down the pushrod or oil drain holes and see if it has the roller cam provisions. the best places to look are in 87 and newer passenger cars or 96-99 trucks and vans with the vortec motors. those all had roller cams and one piece rear seal blocks.
also- make sure it is a 350 and not a 305 by looking at the top surface of the bellhousing. a 305 will say "5.0 liter" and a 350 will say "5.7 liter" in raised cast letters. and you want to make sure you aren't looking at an LT1 block- when they are stripped down to a short block, they look a lot like a regular small block, but the front of the block looks completely different due to the different water pump setup on those compared to a regular small block.