If you build a 302 you can have it shift at 6k easily, it just needs a better camshaft/heads/intake, and exhaust. The SOHC has over 9.5:1 compression which is why it makes better power per C.I., but better 302 pistons are very common. The cost to upgrade those parts is definitely high, but it'd be much more to upgrade those of the 4.0 SOHC. SBF parts are really popular and common, even with the parts delays. The project I'm going with will be a 337 SBF, and shift in the 6k range; with a 2.6 liter blower, it won't need too much rpm.
A member of the Corral posted a link to a modified Porsche that shifts as high as 11k, that was a sweet video. Here's a 350hp car that weighs 1860lbs;