It's not always just about ET, for example you can run 9s with a 1.6 honda civic if you want to turn up the RPM and strip out enough crap but it would be more drivable and fun with a mild 440 swapped in.

Also the age old question about "is there enough cylinder head" is always relative, an X head flows nearly the same as a 906 head butt plenty of guys make do with 906 heads on a 440 butt an X head or magnum "won't work" on a 426 ish SB? It will work just fine for the same ET butt be more civilized. I saw a 273 2bbl run 10s how come we all don't just do that? Well because a 340 does it easier a 360 even easier than that 408 easier yet...

A 4.25 in a SB is just as easy as a 4.25 in a BB, I would say go for it. However if you can't get a 4.25 right now and want to get to work your not going to miss out on too much VS a 408 unless your car is extra heavy. My favorite street motor I ever had was a 408 in my 99 dak RT, a WOT down shift would haze the tires at 70mph, it had insane power everywhere in the power band. Low speed was up big time VS the 360 with the same top end and there was no loss of power at 6000RPM vs the 360.


Bottom line, a 426 SB is awesome, a 408 is nearly as awesome 360 ain't bad either butt the smaller you go the harder it is to keep it sane.