Well thank ya Good Sir Herb!
I'd use any of three products, all of which are great for this sort of thing. In no particular order because they're equally effective:
-3M Windo Weld Ribbon Sealer, aka "butyl tape", p/n 8611
3M Auto Bedding & Glazing Compound, aka "liquid butyl", p/n 08509
-Fleetcraft Pure Silicone Rubber Sealer, aka Manus Bond Clear Silicone Rubber Sealant Adhesive.
Each of those have their own minor challenges to the unfamiliar however that's a bit of an overstatement in a way. None are all that complicated or prone to catastrophic failure but if not installed or used properly they'd be ineffective.
All three are available through Amazon for those not involved or having access to general auto repair suppliers.
The most important aspect of cutting holes in the body is to very thoroughly treat and protect the raw metal edges created by the cuts. I'd strongly suggest a small can of POR-15 used strictly to their installation instructions in order to get the best results. This can't be stressed enough, wish the van converters of long ago had paid attention to this.
Feel free to ask any questions before cutting begins----can't back track one that's begun!