I found this thread through google looking for 5.3 deck heights. Why this showed up I have no idea but I have something to add to this even though I'm sure you have figured something out by now, I'll offer these suggestions.
1. All factory truck and 99-02 fbody manifolds are very large and boxy because they are cast iron
2. Factory 98 fbody manifolds are actually stainless steel. So you could cut the flanges off and weld an extenstion to it.
3. Vette manifolds are well,,, Vette manifolds and their application and fit can almost always be questioned.
In case you guys didn't know, the Ford 302 exhaust port spacing is exactly the same as the spacing on a GM LS motor. ford headers to lsx - LS1TECH
Thus, what alot of guys in the LS world do with swaps is they purchase a set of 302 headers that will work and cut the Ford flange off. Then purchase a set of GM Performance LS flanges and weld them up GM Performance 12480130 - GM Performance Header Flanges - Overview - SummitRacing.com
. You have a much larger pool of vehicle types and header configurations to compare by throwing a 302 Ford into the mix. Obviously the ford ports are rectangular so you will need to oval them out with a pipe expander or something like this with some heat
Also don't overlook the possibility of modifying a shorty header off an Fbody to fit like I did.