273 to 340 swap in 64 cuda auto advise needed
The diagram in post #7 is not correct for a 64-66 273 bracket. It's in the Mopar Performance manual since forever, but it's wrong. That's a 67-up bracket. For 64-6 bracket, just fab a triangle that matches the opposite side and weld it and you'll be good to go. Pic:
Driver's side bracket, you can fab a spacer, or just use a few washers. Viz:
If you use the 71 340 water pump, pulleys and brackets, you'll have to move your lower radiator hose opening to the opposite side, and you won't have room for a clutch fan. Or - just use the 273 timing cover, pulleys and water pump and keep your original radiator and you'll have room for a clutch type fan. The original 273 water pump used a smaller pipe for the crossover hose, but modern replacements are machined for the larger 340 style hose, with a spacer to use for the smaller 273 hose.
As for exhaust, I've run 340 manifolds in the past, both early and late. Late (i.e., 71-73) center dump fits quite well on the passenger side, early barely clears on that side (and may not on some, given dimensional vagaries back in the day). Driver's side interfered in my car with the steering joint. Doesn't in every one. I fixed mine by using a manual steering gear box, power steering column, and an adapter to mate the two. That moved the joint out of the way. Since you are running a Borgeson box, you may not have any clearance problems, don't know, never tried that. Routing the exhaust downwind of the driver's side manifold is another issue. In my 4-speed car, the only way I saw was to run the pipe between the torsion bar and the frame. Others (perhaps with automatic trans) have successfully tried a different route.
I have TTI headers now, but I can tell you that installing them is a huge job, given the various things that have to be moved out of the way (like fuel and brake lines, wiring, etc) and clutch linkage clearance issues. Can all be fixed, but they are not a straight bolt in. Also not cheap, but I sold my early hi-po 340 exhaust manifolds for enough money to pay for the TTI's. Your 71 manifolds probably aren't worth as much, but could still defray much of the cost of a set of headers, TTI or Doug's.
The diagram in post #7 is not correct for a 64-66 273 bracket. It's in the Mopar Performance manual since forever, but it's wrong. That's a 67-up bracket. For 64-6 bracket, just fab a triangle that matches the opposite side and weld it and you'll be good to go. Pic:
Driver's side bracket, you can fab a spacer, or just use a few washers. Viz:
If you use the 71 340 water pump, pulleys and brackets, you'll have to move your lower radiator hose opening to the opposite side, and you won't have room for a clutch fan. Or - just use the 273 timing cover, pulleys and water pump and keep your original radiator and you'll have room for a clutch type fan. The original 273 water pump used a smaller pipe for the crossover hose, but modern replacements are machined for the larger 340 style hose, with a spacer to use for the smaller 273 hose.
As for exhaust, I've run 340 manifolds in the past, both early and late. Late (i.e., 71-73) center dump fits quite well on the passenger side, early barely clears on that side (and may not on some, given dimensional vagaries back in the day). Driver's side interfered in my car with the steering joint. Doesn't in every one. I fixed mine by using a manual steering gear box, power steering column, and an adapter to mate the two. That moved the joint out of the way. Since you are running a Borgeson box, you may not have any clearance problems, don't know, never tried that. Routing the exhaust downwind of the driver's side manifold is another issue. In my 4-speed car, the only way I saw was to run the pipe between the torsion bar and the frame. Others (perhaps with automatic trans) have successfully tried a different route.
I have TTI headers now, but I can tell you that installing them is a huge job, given the various things that have to be moved out of the way (like fuel and brake lines, wiring, etc) and clutch linkage clearance issues. Can all be fixed, but they are not a straight bolt in. Also not cheap, but I sold my early hi-po 340 exhaust manifolds for enough money to pay for the TTI's. Your 71 manifolds probably aren't worth as much, but could still defray much of the cost of a set of headers, TTI or Doug's.