No adapter plate needed. That transmission should bolt right up to the bellhousing with the right clutch disc. I'm not sure if the driveshaft slip yoke on your 58 will mate to the t5... You may have to swap the slip yokes or even have a new shaft built.

I see other threads about putting a v8 in the same truck with a th350... Are you keeping the 270, or swapping to the 350/350? Perhaps you are gathering all the information before making a decision. Personally I like the gmc 6 motors. However my 59 gmc 1/2 ton longbed was born with a v8 from the factory. It sure is fun to drive when it is running. The stroked 1960 pontiac should be out of the machine shop this week. I can't wait to start smoking tires again!