You could use regular gas,depends on your camshaft selection w/the raised compression.
Right now,you probably have between 8.2 - 8.5:1 compression w/a stock head now.
A TH400 trans is not really a disadvantage over a 4 speed. More consistant,1/4 mile runs,easier to drive,etc.
Yeh,the 3 speed hurts/kills the pull when you shift into 3rd gear,rpms drop too much,I know ,I had the same trans when I bought my car. There are different ways to help your 250 run better, but you will need to sacrifice something most times to gain something.
Like if you want more torque out of it,raise the compression,(might need to run 89 octane) more top end,camshaft,keeping the engine closer in the torque band,5 speed.
292 would be nice,if you want a torquey stump puller.But,how much does a 292 weigh over a 250? Adding weight also slows down your acceleration.
