:=what size is the rear end on a 74 comet gt and will the rear end and 3 speed trans take over 300 hp

It should be 8inch, 5 lug. Yes, it will, I would guess that the trans would give out before the rear end depending on what and how you are going to drive it. Others will tell you to go to a 9inch, it is a lot stronger but if money is involved with your decision, keep what you got. Have been running an 8in in my 78 Fairmont for several years, 4:62 gears, 28 x 9 MT slicks and spool. 302 engine at around 325HP or possibly more. NO problems! Did put a shortened 9in under my son's Mav and used the original 8in axles, 4:86, 10 1/2 slicks and over 400hp. Probably have over a 1000 runs in past 8yrs, NO problems! But it's up to each individual to decide what's best for his application. If I did not come from the backyard mechanic's era, I probably would just order a Currie or Strange 9in rear set up, but old habits are hard to break. Best of luck either way.