As far as geometry goes, if the ET is the same, the outsides of the wheels will be 1" nearer, so your track will decrease by this amount.
However this will only really be visible if the tyres you fit to your 4.5's are narrower too by a similar amount ie dropping from 185 to 165 or 155. Although I'm guessing that your going to go narrower.
Narrow tyres on narrow rims look very good in my opinion. I'm not a T2 owner, but I've seen loads of buses with 145's on.
As for safety, you'll have wait for someone else to reply. I know that the weight rating on non-commercial tyres is never really up to O/E spec (100-102)
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