help please

Welcome to Stangnet!

To answer your question, it really depends on what you want to do to the car. If you're looking to turn around and sell it in the near future, then i'd stick with the auto. If you're planning on keeping the car for a while, and you want a manual, then go for it. I would avoid the t5 behind such a large motor. the toploader would be a good choice. If you want 5 speeds you can look into the tko and the richmond transmissions. Sometimes the TKO requires some modifications to the transmission tunnel, but i'm not sure if that applies to '67 cars. Prices vary depending on which transmission and clutch linkage you use. the toploader would probably be the cheapest. If you want more information you can run a search of the classic section and come up with some great info.

Personal preference...which do you want? No way a T-5 lives long behind a big block, so it's either the toploader (no O/D) or a TKO ($$$). Typcially autos will handle a little more power and take it a little easier on other driveline components because the torque converter absorbs some of the rough stuff (at the expense of a little more power than a clutch/manual). You can always fiddle with the auto to firm up the shifts if you're after performance.

Get an AOD. Better low end launch and better gas mileage.

Cost to convert to a manual will depend on how good you are at parts hunting and how patient you are as well. If money is no object, the swap can go upwards of 2 grand or more. Used Toploaders go for as little as $300 that need repairs ( they're easy to fix yourself) I also agree with the others, forget the T-5, a 428 will shread one in short order. A small block Topoader will do fine for most, but if you're into the drag race scene and put sticky tires with posi traction, you can break one too, but it'll take the right amount of traction with the torque of the 428 to do one in. I ran them behind a 427 on the street without problems, but the guy who I sold the car to, broke it on the drag strip after adding sticky tires and a button clutch. He broke the 9" Versailles rear too in that episode. For a 5 speed look at the Richmond or the TKO 600 series.