Shorty headers will not work, either get stock manifolds that exit at the rear (better anyways), or get headers that exit at the rear. The S-10 swap headers are made to fit for an S-10 body and steering setup, not a '53. Looking at the swap parts isn't the best idea, since your motor and trans may need to be in a different spot for the '53.
There is no need to change the oil pan. I have a 1982 S-10 with a 350 and have never heard of this. The stock oil pan works just fine.
When you're putting the motor on the frame, don't assume that the swap plates to put a 350 in an S-10 will put it in the right place. Measure from wheelwell center to the firewall, then you'll know where the back of your engine needs to be in reference to the centerline of the wheels.
Motor mounts are fairly simple to fabricate, but you do still need the rubber side for it to mount properly. I just installed the 455 Buick in my S-10 frame for my '46 GMC I'm currently building last weekend, and I fabricated my own mounts.
