273 302 318 340 build

If the same pistons are staying in the 318 block then I can suggest:
- Use a Comp XE or Lunati Voodoo or Crane cam of shorter duration; you will get some higher lift than stock lift to help the mid range and top end, with the best lift to duration ratios you can get.

With the existing flat tops and the 64 cc 302 chambers, your SCR will be at 8.6 with the 1121G head gasket. Mill the heads .040"-.050" and get the chambers down to around 59 cc's, and now you are at 9.1 SCR.

All of the following assumes you milled the heads to get to the above SCR. Some cams to consider:
- Put in a Crane H-260-2 hydraulic as the largest duration cam to use with 6 degrees advance (1 degree more than the 5 degrees of ground-in advance), and you'll be a 7.7 DCR... considerably better than you were. (I like the Cranes for their less aggressive ramps that put less peak pressure on the lifter/cam contact.)
- Put in a Voodoo 10200701 (installed straight up with 4 degrees ground-in advance ) and you'll be at the same point with a hair more lift.
- Both cams have 'little brothers'; the Crane H-248-2 (installed straight up at its 5 degrees of ground-in advance) would push up the DCR to 7.95.... now you are closing in on 'as far as you should go with iron heads' without and still keep the tuning relatively easy and run pump gas. This cam's lift is only .400" on the intake; easy on the valvetrain but I would bet it will cut your usable top end RPM's by 800-1200 RPM's over what you have now. I am not sure if that is what you would like or not.
- The Lunati Voodoo 10200700 will give you back some lift and take away a bit of DCR versus the Crane H-248-2. So it will move the RPM range back up some. The ramps are more aggressive so more care with the valvetrain is needed.

Since these are hydraulic lifter cams, I would expect different pushrods for the 273 rockers. (And I have not run numbers with your present cam with more advance and maybe a bit more lash. But I don't think that's gonna get you all that far.)

FYI, I have run a low duration cam like the H-248-2 with higher rocker ratios with a good breathing heads/intake/headers, with 10+ SCR, and had a solid 1500-6000+ RPM engine with great torque, over a ft-lb per cubic inch. So a wide torque range on the street is not all that hard to get if you focus on a high SCR to start with, and then work on the breathing components, and get the valve's lift to duration ratio as high as you can. Others have done the same so this is all known. You won't be in 400-500 HP that way in a small block, so it is not the formula if you are into lowest ET's....but it has superb driveability on the street, and is a versatile type of engine combination.

And yes on the adapter for a pre-'68 TC and a '68 or later crank. (I have never run into this but everyone says it is needed!) You can throw the calipers on both parts to be sure.