1999 Miata 1.8L engine w 5 speed trans. vs 1999 Miata 1.8L engine w 6 speed trans.

I blew up my 1999 1.8L engine w 5 speed. Always aware of the needed one more gear
while driving my car. In my replacement engine search, I found a Japanes low miles 1999
1.8L engine with 6 speed transmission attached in America. I have talked to a number of
Texas Miata people but cannot get a definitive answer as to whether the 1.8L with six
speed will fit right in without complications.
One parts man noted the vehicle speed sensors are different between 5 and 6 speeds. He
was unable to tell me what the consequences of that might be....wrong speedometer
reading or no speedometer reading ? Other possible issues noted: transaxle ratio etc
might not give you the performance you are expecting. Other issue drive shaft (length) ?
Other possible issue: computer monitor/switches, wiring might be different. Any thoughts?