On the gearing, I think your feel is correct. From experience, with a 4.1sh diff, a 2B series and a LWB, the 5th gear does not gain anything much on the top speed, but is more of a relaxant if you are cruising on the highway or the motorway. Though, a good 3B and a SWB may be able to gain you a couple of more relaxed kmphs on top speed, but not much I would think. Its a rather lost cause anyways: BJs and topspeeds
One thing you should look into is to avoid variation in 1st to 3rd gear ratios between your current H41 and any H55 which you would swap in. I may be wrong but I came across some variations in the H41/42 and H55 gear ratios last time I tried to look into this. Would perhaps be good if someone can post the rundown on these variations.