Will a 1994 Chevy truck transmission fit 1988?

No, a 1994 Chevy truck transmission is very unlikely to fit a 1988 Chevy truck. While both are Chevy trucks, GM made significant changes to their truck platforms and transmissions throughout the late 80s and early 90s. The bolt patterns, input shafts, output shafts, and even the overall case dimensions are likely different. You'd need to compare the specific transmission part numbers to be sure, but a direct swap is highly improbable without extensive modifications (which are usually not feasible or cost-effective).