What transmission could I use to swap with the 3-spd unit in my 81, 1.5L? I'm scared to drive it over 60 as it raps up so high. What are my options to make this thing capable of interstate speeds without blowing the darn thing up? Open to tire size changes as well, if they will make significant improvements. Would need to know how to remedy speedo discrepancy issues in that case. TIA.