Sequential system reducers can be mounted in any orientation, though there may in theory be slight advantages for longevity if they are installed in a certain orientation, would depend on reducer type and where inside it heavy ends are likely to build up. But since any slight advantage here would be completely overshadowed by the negative effect on reducer longevity of allowing the reducer to become a block of ice, it is better on vehicles with high risk of airlocks to mount the reducer in orientation that will best prevent airlocks.
I use loads of different types of filters on new installs, the handiest types are usually the smaller types, if I get too many of the big types in stock I'll fit them on vehicles I'm converting that have loads of space! The big type probably contains a gauzy type filter, reckon the small paper type do the best job of filtering but will need changing more often. Probably no advantage to being anorak about type of any filter, biggest job they do is prevent big bits of crispy heavy ends clogging injectors (vapour) or bits of foreign stuff from clogging reducer (liquid).
A new engine wouldn't put me off running it on LPG, but I'd be 100% the LPG system was working properly.
What to do with a new engine with a new cam... bottom ends like to be run in gently at first, cams are often better run in at high rpm. But nobody is going to fit an old cam in a new engine and swap the cam for a new one when then bottom end is run in, not in a pushrod engine anyway...
Simon