What year mustang engine do you have? Starting early in the 80's the dipstick tube goes through the block on the drivers side just above the oil pan rail. Either use this or there will be a hole spraying oil everywhere.
OK, so you have an early block without the hole. Maybe, we don't know, didn't provide enough information up front. Now another question is what pan do you have? Those early mustang engines ran a front sump pan, that won't work in a Bronco chassis. Best to find a Bronco oil pan with a dipstick at this point. Really hard to guess at what you need since we know so little about what parts you actually have.
But you did ask a specific question I can answer. Will a 351 pan work on a 302 block. That is easy, NO. The bearings on the 351 are larger, the rear main bearing is larger, the pan has a larger radius to match that larger cap. All the bolts will bolt the pan in place, that isn't a problem. The gap you can stick finger in at the back of the pan is the problem.
So what year 302 are you working with?