I recently aquired a new inline 3400 MAF sensor. Took it to the garage and it plugged right in! It is much bigger than the TB and was wondering if it will:

1: will it work?
2: does it need ECM tuning to work
3: help performance
4: If I were to use this MAF and make a plate to block off the MAF sensor opening would I need to block off the whole above the TB inside the TB that the orginal MAF sits in.

IMO it wouldn't be worth the work. Your defintiely going to have to tune it. If it'* bigger than the TB, your going to be loosing velocity because of the changing diameters.
Your best bet would be to grind off half of the bottom half of the maf post for more flow.

I have a stock MAF and a 3400 MAF that I am testing on the flow bench for the very purpose you mention. Yes the connections are exactly the same, (at least compared to a 1990 Hitachi MAF), so it does plug in and operate. From the limited testing I have done so far (only the upper flow ranges), the 3400 MAF flows 1.52-1.60 times the amount of air at the same frequency output, so retuning will be needed. The 3400 MAF is much larger, but, the design is "dirtier" than the stock setup. Conventional wisdom says for maximum performance, the inlet restriction (depression) should be 1" of vacuum Hg, (14" of water), or less. At that test pressure, the stock '90 MAF flows approx. 325cfm, and the 3400 MAF flows 520 cfm. This is also very close to the point that both MAF'* reach the maximum frequency of 10.4kHz the stock ECM can read. My purpose is to reduce the intake restriction for the turbo setup (MAF ahead of the compressor), and also widen the range of air flow the ECM will be able to read, with proper tuning of the tables. In a non-boosted application, the larger MAF will probably be of very limited benefit unless the horsepower output is somewhere north of 200 hp. Essentially, unless the diagnostics indicate the MAF is maxed out, little will be gained. If you do decide to replace the MAF, the best would be to remove the stock one and make a new adapter to attach to the throttle body. A long smooth taper from the intake size of approx. 3.5" at the MAF to the diameter of the throttle passage will help build velocity and that may give a small ram effect, at least in some rpm range. I am in the process of making smaller pitot flow tubes to read the lower flows with better accuracy to see if the approx. 1.5:1 ratio of air flow to frequency is true for the entire range. At this point I can only read down to about 5kHz with reasonable accuracy.