In terms of the 200-4R, I didn't see any mention of dealing with the lockup torque converter and the TV cable. These two are VERY IMPORTANT, particularly the TV cable.
The lockup is electric, controlled by the computer. For non-computer apps you'll have to run a switch in order to lock-up the torque converter. You can check Summit, Jegs for this kit.
The throttle valve, or TV cable, takes the place of the modulator on the trans. Miss adjust or don't use it and the transmission will shift like crap and won't have a long life. Not sure what carb setup you're going to have but you're going to need this.
You can also check on getting a G-Force crossmember, especially if you're planning on doing a larger-than stock or true dual-exhaust. It's really beefy and has cut-outs built in for the duals, really helpful if you're running headers. The issue with the stock crossmember is that the pipes can hang down lower than wanted, particularly the driver side, when running a true dual. If you going to run a stock-type dual system you don't really have to worry about it, as the exhaust chokes down via the y-pipe into the converter then comes back out.
As stated you should be ok on a stock x-member since you have an '84, just take a quick peek at the driver's side frame-rail. There should be two sets of holes. The ones more forward, toward the front wheel are for the 3spd trannies: TH350 and such; the second set is for the TH2004R. GM angled the x-member back on the driver's side for the 200-4R, hence the second set of holes. Make sure you have the second set of holes in the frame-rail.
Hope this helps and good luck.
