The front access is for water temp sender and maybe heater return. The front ports are going into coolant.
The rear port that you describe would typically be for vacuum brake boosters. However, I've got two Astros (94 and 97) and they both have systems that use the power steering pump as brake assist. Not sure about '02 models...?
You may have a vacuum brake booster, but if you don't have a big vacuum booster cannister, you probably have hydraulic boost. If so, you don't need vacuum for the brakes.
Looking at the master cylinder will give it away...if you see two large power steering looking lines going into the booster, you got hydroboost and don't need vacuum for the brakes.
If you have a big vacuum chamber behind the master cylinder...well...you can put a large "T" into the rear intake vacuum port and hook it all up.
These sorts of engine changes often require moving coils, which you can mount anywhere...even on the firewall above the engine (near the wiper motor), and fuel lines are easily rearranged.
Check your master cylinder and be sure that you really need vacuum. If you have a vacuum booster, I'm gonna find one and modify mine. I have never been a fan of the hydraulic boost units. I like the feel of a vacuum powered unit.
If you have questions regarding the style booster your van has, post a pix of the master cylinder and it can easily be identified.