short block: the crankcase, crankshaft, con. rods, camshaft and all block 
seals. hopefully an oil pump

long block: ditto plus piston/cyls/heads/valve covers, full new 
seals/gaskets  NO auxilliary stuff

complete engine: ditto above plus auxilliary stuff like fuel system, 
electrics, tin ware, exhaust system, flywheel, clutch, engine mounts

short block requires many $$ of new components to complete into a long 
block, and some modest mechanical skills/tools. It's not likely you needed a 
new short block without needing the rest of the engine freshening up also.

long block requires you bolt all the old rusty exterior bits of your current 
engine onto it to get an 'engine', minimal skill/tool level requirements

complete engine requires you bolt on a suitable bus before you can motor 
around in comfort.

Having done long block, short block and complete rebuilds on bus engines the 
way to go is definately a long block, barring $$$ limitations. Warranty on a 
long block should be better, since 'they' know what the engine will be doing 
roughly, while a short block allows you to go to non-stock barrels/heads etc.

  Beware the combo of old short block and new p+c and reworked heads. You 
get good restoration of power etc., but the new loads may be too much for 
unrestored crankcase bearings and they may start to thump during your new 
found hard acceleration. Check the short block first before rebuilding onto it!

tim s.   

ps..  a '91 and you need a new motor??? my '85 has over 100K miles, and 
performance is still A1 on hills/highway, just passes water like an 
incontinent wino.