That 3.42 rear end might be great behind a 1:1 ratio transmission, but like Hotrod Lincoln says, it would render the fifth gear of your T5 useless. I've got a 3.90 rear end, .72 overdrive T5, 29" tires, and a stock 216 and I've run my truck at 70-80 mph on 4 and 5 hour trips many times.
If you don't mind me offering my opinion, I would suggest keeping either the T5 with a rear end ratio closer to 3.90, or the 3.42 rear end with a trans final drive ratio closer to 1:1.
