I'd really like some pics of these V-8 installations, especially showing the exhaust and crossmember hook ups. Naturally an under hood shot! I have some, but more wouldn't hurt. Please send to my e-mail (email@example.com) even if you post here.
One I've seen used rear dump exhaust but switched side to side (lucky you can do that on a Chevy!) so they exited the front. That was done to clear the stock steering. The engine can be moved forward a bit to keep the heater -- it won't be any more nose heavy than with the heavy old stock six (which isn't much lighter than a modern SBC!).
The old L-head just has a 3.125" bore, that's why it's so short (4.25" stroke if it's a 195.6, older 184 and 172.6 had same bore, shorter strokes). No other inline six is that short, maybe a couple of the olde L-heads from way back, but nothing modern. The 60 degree V-6s are the best bet. I've considered a Caddy 4.1L myself, just to get a cheap aluminum V-8. The light American doesn't need a whole lot of power to be a real hot rod!!