Backfire under load

[70GMC396]

OK, need some help - think it is just an old carb in need of some TLC. It is a 4MV on my 402 auto 70 GMC. Bogs down quickly and backfires each time I try to accellerate on the freeway about 80% to WOW - don't think it is WOW only, but could be. I let up and smooths out, but will bog down and backfire again if I try to recover. Can really only accellerate at about 50% throttle most the time reliably. It hasn't always been this way - father-in-law owned it since 1980 and confirmed that, but I resurrected the truck after it sat for a few years. I am due to replace the fuel filter at the carb and a rebuild wouldn't hurt as it heavily varnished and leaky. Any thoughts on what to check first? Also a vacuum powered valve towards and on top the left front of the intake (anti-back fire vlave or something else???) has a loose electrical connector - looks expensive to replace - could this be related? THx