Yep that's pretty much what I did when I changed to clear reflectors a few years ago, use a heat gun though as I melted one of my lenses when it touched the oven shelf! Also use some sealant around the outer edge when reassembled just to make sure.
I'm sure you can get behind it from inside just there so I don't see why not. Call a PDR company and get them to have a look at it, they'll soon tell you.
With a free flowing exhaust, plenum spacer and velocity stack air box around 10-20 hp at most. You probably won't even feel it on a 350. If you want a pop charger then go for it , it will lose a couple of horses at most .
5w30 is fine but not from asda unless it's a real emergency! The dipstick is awkward on the de , best left over night on level ground to get a real reading then check it a few times. Oil level on these engines is very important, don't run them low!