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If you look at the transmissions, you will find that there is NO overdrive. Throw in a 700R4 and you'll see 140+. Until then, lots of redline, not much speed.
Well I've got a pretty unique perspective on this if you want to compare an 86 Z-51 auto to a 73 LS-4 close ratio muncie, since I drove both cars to Effingham this weekend (different days obviously). The LS-4 doesn't have any problem dispatching the L-98 in a straight line if it can hook up, the amount of torque difference is substantial, and the L-98 is an awfully torquey small block but the 454 is a different world on the torque table. Now the 86 Z-51 is better in just about every other category however, especially handling. Now 74 is slightly different then 73 in that the LS-4 is rated 5 horse less and these are net ratings and have been since 72. Now comparing small block to small block, it would take an awfully well optioned 74 L-82 with the optional gears and somebody good with a 4spd to take an 86 Z-51 with 3.07's. If I had to bet between those two I'd go with the lighter 86.