If you could start with a 6.0L but you really aren't going to find one of those cheap unless it needs a complete rebuild. Even then a LQ4 is only 300 hp and 360 TQ. When you start adding more cam to it, the already weak low-midrange torque only becomes even more noticeably gutless.
I have a LQ4 with ported/milled 5.3 heads, a 220/224 cam, headers and a trailblazer SS intake in a 1987 G20 van backed to a 4L80E and 3.73 gears. Once it gets into the 3,000+ rpm range it moves pretty good and continues to make good power up to about 6,500 rpm. In the lower RPM range it is a pig. Its no better on gas than the 350 in my Express van and actually worse with a trailer behind it.
Yes, I agree a 6.0 is going to be a much more expensive starting point because they’re all the craze right now. I was talking about the 4.8/5.3 platform. Sure they definitely like to rev, but he’s coming from a 305. You also took an already rev happy engine and moved the power curve up even more with the your mods, and are comparing it to a truck with 5.13 gears. It’s not really apples to apples.
I’ve had several 6.0s in work trucks and have never had a major repair. I have a 2012 6.0 now with 180k hauling right at capacity with a CAT 259 skidsteer around every single day for the past several years with no hiccups. Zero. One set of plugs, a front end rebuild, and fluid changes, and it does fine. It takes a little throttle to get moving and doesn’t pass many gas stations, but my 3/4 ton 350 Suburban doesn’t even like OD on flat ground at 65 with a 20 foot bass boat behind it. And in 122k miles it’s needed intake gaskets, injectors, a distributor, took the heads off for a valve job and found both to be badly cracked with zero instances of overheating since my dad bought it new, consumes a ton of oil, etc. I love the 400 platform, that’s why I’m here- but I don’t think that the stock small block does anything better than the newer platform engine-wise.