One item about the use of a late design filter in the early trannys..... your valve body ( if it is early unit) will NOT have a threaded hole for the late design filter retaining bolt.
The late design filter uses a shouldered bolt to hold the filter to the valve body...... although the filter will move a round a bit.
Second........ the late design filter uses a separate pick up tube with O-ring that goes between the case and the flat filter. This tube would NOT be in the gasket / filter kit you bought.
You may have to find a local tranny shop or parts supplier to purchase the early design filter for your t-400
Dont worry how old you are... it is the mileage on your body that adds up.