| Actually they both should be 30 spline on v8's.
The differences range from more vanes in the pump and other pump mods to larger sprags, hardened planetaries, a better valve body.....
If you have a choice go with the 88. If the 88 trans is shot put the 86 in for now and do/have done a good performance rebuild on the 88 if your're planning on putting serious power through it.
As for bolt up differences, they are alike.
If you need to determine a good trans from a older (weaker) trans, pop the pan and look at the valvebody. There should be a metal tube running from the pump to the rear of the case (into an aux valvebody, made of aluminum) The older trans had a 'blockoff' plate installed at the back. That's prolly the easiest way to tell, but a later tranny thats shot isn't better than a earlier model in good condition.
Hope this answers your ?'s
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