Belltech for the flip kit, hands down. Belltech shocks are good. I'd also go Belltech for the front springs and spindles, but everyone likes to dwell on saying the Belltech spindles push your wheels out (1/2", 1" whatever) I personally am not convinced but have never had the chance to compare different brands in hand and take specific measurements. I've run a number of trucks using Belltech spindles and as long as you pick your wheel and tire correctly there is no issue. They are by far the best quality stuff you can get.
The most important points to having a 4/6 (or even 5/7) drop and still using it as a truck are...
1. c-notch for the frame, absolutely critical, you will not get away without doing this as there simply isn't enough room for the axle to travel otherwise. The truth is even with a 3/4 drop you will need a notch if you're going to carry a heavy load.
2. correct tire size and wheel size so you don't get rubbing on the front. Front wheel width of 8.5" with a 5.5" backspace is ideal for pretty much most tire sizes, of course you need to keep tire height down to a reasonable amount - 30" or so which is not really getting too short on the sidewall until you get into 22's. For example I often run a 255/45/20 up front and it's a great fit. That's a 29" tall tire with about 4.5" of sidewall so you aren't smashing your wheels up on every little pothole.
3. correct load range of tire. Too many low profile tires are meant for lightweight passenger cars. Look for a brand you can get XL rated tires if you intend to haul weight around. I'd pick them anyway just because you have a heavy vehicle that you may be prone to driving around in a sporty style since it's lowered and handles better. With the higher load rating you get the stiffer sidewall which will prevent any mushy handling. I've been using Toyo Proxes S/T for years now, they've been replaced by the STII which also has been good for me. Good grip dry and wet, good treadwear, XL load rating. Plus many sizes are made in USA, some in Japan.
20' car hauler trailer and a classic Thunderbird body are not exactly light
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Neither is a bed full of 19.5" tires and wheels
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Not like those are stupid heavy loads but things I never thought twice about, just load the truck up and go.
Richard