John,
As a reply to your other questions:
(a) Physically, no! But fitting a Mark gearbox makes topping up and checking the oil level somewhat awkward as the filler plug will be under the toolbox. Why not just remove the intake tubes and fit carbs direct on the heads and retain the original gearbox?
(b) I don't believe the Dragonfly has a better, stronger bottom end than a Mark - let alone a Plus. The modified oiling system used on the Dragonfly may give better oiling to the bigends when in good condition, but any subsequent wear biases the oiling toward the front big end, and starves the rear. Eddy Withers had the same problems with the works 'Plus' racers - engines often only lasted for one race! Also Plus/Dragonfly crankshaft centre webs are drilled right across, creating a weakness that often results in the web cracking and loosening the crankpins. There are also other lubrication problems with the Dragonfly caused by the timing gears sitting too deeply in the oil retained in the timing chest - the usual result being that continued use of high revs causes the excess oil to be trapped in the timing gears, thus straining the teeth until they eventually break off (usually from the camshaft idler gear). There is an easy cure - just drill 2 drain holes in the back of the timing chest to reduce the oil level so that the cam gears only just dip their teeth into the oil.
Plus barrels will not fit on the Dragonfly, but the heads can be - if they are modified for through bolt fixing (as on the Dragonfly). This also means the rocker blocks need modifying to clear the nuts on the lower pair of head/barrel studs. To get any worthwhile benefit from the heads, Plus cams will also be needed.
Regards,
Eddie.
