I agree the 4.0 is near its limits in stock form. The building choices for those is nothing like a common engine, such as the SBF. It would cost a lot more for head work for sure, which might be the biggest gains beyond bore/stroke changes. The shortblock possibilities would be harder too, plus likely not feasible. I don't know anything about the 4.0, I expect there's not much beyond rod and piston quality upgrades. The heads may have room for good porting, slightly bigger valves, but custom cams would be a big item to want. Those as a final touch would be worth it, hopefully along with gains in the rest of the engine. If any of that is possible, I could see $5k being the cost. That's a lot for an unknown result, and the transmission is still a weak link.
I wouldn't do it given the transmission, that alone is why I want to V8 swap my mail truck. I don't trust it in a mild long term application.
Dumb question, what are the dimensions of the late 90's truck V6's, the 4.2 I think it is? Those came with a 4R70W behind them, the same 302 SBF version. If that fit, it could be a good platform to use, if the heads have flow potential like aftermarket SBF heads(say 250cfm and up). I think stock SOHC heads are likely not much more than 200cfm, but what's needed is in the range of 300cfm, for a feasible cost.
SBF heads you can buy for $1500 out of the box like that, but to port lesser heads to get there, can be $500 and more. What I have to build my blower project cost me $1800(ported 170TW heads), which flow 315cfm @.600 lift. That's a good price for that flow level, but can that be done with any of the V6 heads?
