The L/R band has no effect on the 2-3 overlap.
A leak out the bellhouse can be misdiagnosed as a pump leak, when it can just as easily be a vent leak.
If in fact your 2-3 issue is overlap, and I think it is, Then it may be either a too-tight KD band adjustment, OR, as you are thinking insufficient springing in the hi-drum(front clutch). It could be other things too.
If the tranny whams into reverse at a normal idle speed (750ish), then perhaps someone has removed the wavy spring from the hi-drum. It is there to cushion that N-R shift and also to cushion the shift into hi,by slowing it down at low line pressure,low throttle pressure. With a reverse manual shift kit, this is easy to prove by shifting from Neutral into Direct, at idle. If it whams in, the wavy spring is gone. Sometimes this wavy spring is eliminated during the front modification to install extra plates into that drum.I never do that, cuz I hate that harsh shift into reverse. If the wavy spring is gone, you will need to compensate for that with extra spring pressure.
At this point I should bow out on account of I haven't done a ton of these conversions for other people. During my learning process, that 904(yeah I like 904s) was in and out a lot. I was young and strong and excited to learn this stuff; you may not be....heehee. Plus I had free access to a hoist.And a jack,etc. I don't like RMVBs on the street; but that is a personal thing.I like redheads;and my blond wife is somewhat accommodating.Yeah that's personal,too.