I'd definitly do the disc swap first. Always best to be able to stop and steer the car before being able to roast the tires (and upgrading the rear end will better enable you to put down that new found power :D )
I picked up a '65 GT last summer and found on e-bay a full front rotor set-up off of a '65 Barracuda the same week. Won it for around $350. and put it all together while my 273 2bbl. was being rebuilt to commando specs. I was super pleased with the install until I ran into a roadblock to get the "perfect" power booster which took me over two months to find. What a score (and relief) that was. Any other workable (small coffee can looking instead of the correct larger sort of sideways cake looking correct one apparently do not have enough air capacity to work properly. Also it isn't easy to find the tandem master that comes on a swoop up bracket to clear the horizontal hood springs. If you've got the doe though, in Mopar mag., a company out of San Jose, CA 'MagnumForce' always advertises full modern conversions.
