small block to big block chevy

I'm not a tranny specialist but I think  those stock #'s for a th350 and 400 is correct I've seen a th350 stock with a tranny cooler hold up to a fresh 454 with no problems now the rest of the 350 running gear ujoints and shafts either twisted or broke. I would definately go for the 700r4 for gearing a fuel mileage you will want the 31spline later years 700r4 87-92 were the years I believe  

well the thing that kills the 700r4 at times is the overdrive and people running way too high axle gears as snoman is always advocating.. but if you put 700r4 in od off position it would be same as 350 except for the fact it has a deeper 1st gear which would help with tall axle gears, but a good cooler, correct axle gears a 700r4 will do everything a th350 will, just remember the earlier 700r4s up to about 85 had design problems but those can now be fixed and updated to the modern tranny design and yeah its true the 700r4 can be built to handle like 700 hp, now how bad ass is that cabability to handle 700 hp and OVERDRIVE!!!! whats not to like????  
solid axle chevys cant be beat
78 K5: 350 V8,700r4,np208, 8 lug D44 4:88, 14 bolt 10.5 4:88 with detroit locker and 35 inc parenelli jones dirt grip tires

Now you are starting to think right about this. Now add the fact that you CAN run some seriously deep gears (up to 30% deeper)with a 700R4 for better performance and still cruise at a reasonable RPM and then you have the full picture here.