What are your recommendations for easy and effective performance modifications for the 1990 -94 LS400?
Inexpensive would be nice.

The most bang for your buck IMO on the gen 1 cars is get it down to a good muffler shop, and have them replace the stock mid, or "B" pipe, replaced with individual tubes, with an x pipe at the recommended location. The stock unit is extremely restrictive, and replacing it I've heard is good for about 25 hp, which will be noticeable. Keep the new center exhaust tubing the same size as the rear exhaust. Aside from that, a free-flow intake would be a good choice; Luxurymods.com list a couple. HTH!