Oh ... don't be afraid of trying a 1rm on squat. If you have to dump the bar, you have to dump the bar. I've done it many times.
I'd just be sure to do the squats in a full power rack so that (a) you can adjust the safety arms to *just* the right spot, (b) you have more room to fall forward, and (c) the bar dosn't roll away behind you and hurt someone else. If all you have are the L-shaped squat racks, that makes it a little trickier imho. If you're doing 265 for 5, you could probably hit 300 for 1. Just be sure you're well warmed up first. 135 x 10, 175 x 8, 225 x 5, 275 x 1, 300 x 1.
The hardest thing when you start going up in weight on the squat is all your stabilizers. It amazes me how much of a difference even 20lbs can make ... especially if you're walking it out from the rack.
In terms of the deads, be VERY careful to keep your form on point. I messed up my back good a year ago training poorly for the deadlift. One false move and you'll be seeing the witch doctor for months.