Rowdy and Mean 318 build

Your tune was way off on the 262* equipped 360. Mostly I think The T-port sync was off. And the 900 idle IN-GEAR!!proves it. It shoulda been happy at 750, maybe less. With a 4speed it woulda been happy at 650/700.
The transition was hard cuz mostly there wasn't any. I bet the idle-timing was 18* or more and the mixture screws were kindof dead. If that is true, then the sync was off for sure.
The 750 is almost too small for a 360. It will be just right on a 318 with a 268 cam. The primaries will work just fine on any SBM. The smaller the better.
Forget 2.94s with any cam much over stock.

But back up the bus.
Something is going on here. If you were happy with the 360,and it's 262 cam and the 2.94s, then why the sudden desire for 400 hp. I mean that 262 mightof made 330, but it didn't arrive until 54 mph with those 2.94s
Mr.D3
You need gears before anything. If you like, put all the 360 stuff on that teener including the 262 cam.Get the compression up. Then get you some gears in the range of 3.23s to 3.91s,the higher the better, if hiway travel is not important. I guarantee you that teener in an A-body, with 3.91s and that 2600TC will be good for ear-to-ear grins! You will want to get the static CR up to 9.8, for a Dcr of 8.2@165psi This is doable. The power will be down some 13%,compared to the 360, but the performance increase with 3.91s will be some 33%, for a net improvement over the 360, of plus 20%. Plus 20% is H-U-G-E !
Your new power peak with 3.91s will arrive at around 42 mph, NOT 54 as it did with the 360.