What engine are you using? My first guess would be one of the New Venture Gear 5 speeds. There is one that has a reasonable 1st gear ratio, but damned if I cant remember which it is. It'd be something like 'NV1500' 'NV2500' etc.
Parts for these aren't exactly common, and with NVG leaving Syracuse, I can't get them. So, call around 1st if you go this route and price out an overhaul kit. If its too big to choke down, buy an extra trans.
If that doesn't fit your bill, you'll be left with an aftermarket TKO. The TKO with its top shift and 3.27 1st is a really good truck transmission...because that was its original intention for the Mexican market (Tremec is a Mexican company). Its a good trans, lots of info out there. But, it might be too pricey for you.
Next up would be a world class T5 out of a V8 Camaro/Firebird. Only good reliably for 250ft pounds....maybe 300