1.5L/90ci Blower On A Slant

Use an intake manifold with equal length runners to equalize A/F ratios in all cylinders.
The intake pictured has a nice design(runners and plenum)
I know your in the mock up stage but the intake is going to determine the location of the blower. Perhaps just buy a 6-71 base gasket for reference.
That 6-71 is heavy, plan on fabbing up some intake supports. Between the added weight and stress of the belt drive, your going to need to support that intake.
Nothing wrong with running a small bore 6-71on a small ci engine.
I recommend a big bore simply because of resale value, should you decide to sell.
The 6-71 will require at least twice as much HP to turn as a M90 would. The 6-71 requires two 600 carbs (again more weight). Dont rule out an M90 with one carb. Keep it simple.