Jus' as a heads up... it's a somewhat common misconception that a jackshaft/shiftkit will enable a higher top speed but with our bikes this often isn't the case - provided the final gear ratio is the same a shiftkit equipped build can't be any faster than a single speed as there's no "extra power" gained from the added friction and complexity, if anything it's ever so slightly slower due to the drag of it's chains & bearings.
The additional complexity & thus maintenance of a jackshaft is often accepted by builders not for a top speed gain, but for improved acceleration and lowered RPM at cruise speeds, making for a more comfortable & efficient ride...
...but if you want absolute
top speed & maximum reliability, a shifter bike just can't quite compete with a single speed geared tall and pedaled on takeoff.
Food for thought