The 45 series tire will fit. I ran a 295/45 (30.5" tall) on my 2wd Blazer (same rear tubs/axle as a fleetside), so there is room. Most guys run a 295/40 (29" tall) out back with a 255/40-20 up front (28" tall).
Have you looked at pics of 18 front/20 rear combos? They tend to look very nice aired-out, but not so great at ride height. Just something to think about. Streeters are a good choice for price and I have not heard anything bad about quality. Good luck!
EDIT: Just noticed you have a '67. 5.5" backspacing on a 10" rim may be too much on the early narrow rear axle. It would fit perfectly on a 71-72 with the wider rear axle. You should shoot for 5" BS on a 67-70.
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