i have 4.10 gears thinking about putting 36/12.50 swamper tires on a 75 ford 400c.i. c-6 203 f/r dana60's   would i have plenty of gear for the trail  

Some with lower standards would say yes but I would say no. In effective gear ratio it is like having stock tires with 3.42 gears. With stock tires it would run well with 4.10's but that would not be the case here with 36's.  A 4.56 would be better and 4.88 would be better yet but if you do not have a OD tranny, a 4.56 would be a better compromise than a 4.10 here but still it would not run as well with 4.56's as it did with stock tires and 4.10's. Also failure to regear would shorten life of your Tcase. Do change oil in your 203 and do it regularly (every 10 to 15K)and use straight 30 weight non detergent not 10w40 as some suggest.