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!