First off what engine and tranny are in the car right now? Can they be rebuilt? 300 HP is pretty simple to get out of a 302 you might want to hit up the junk yards in your area and find you with a warranty on it to start.
First off you need to make sure a 302 and the T5 will fit with out major work on the frame.
1968 Coupe <53000 miles on her 289 came off the line Jan 21st 1968. I bought it in 1983
2013 Shelby GT500 Born 12/06/2012 number 439 of 530 in DIB and 1 of 9 with Red Stripes over the top and on the seats